Set Partitions

AUTHORS:

  • Mike Hansen
  • MuPAD-Combinat developers (for algorithms and design inspiration).
  • Travis Scrimshaw (2013-02-28): Removed CombinatorialClass and added entry point through SetPartition.
  • Martin Rubey (2017-10-10): Cleanup, add crossings and nestings, add random generation.

This module defines a class for immutable partitioning of a set. For mutable version see DisjointSet().

class sage.combinat.set_partition.AbstractSetPartition

Bases: sage.structure.list_clone.ClonableArray

Methods of set partitions which are independent of the base set

base_set()

Return the base set of self, which is the union of all parts of self.

EXAMPLES:

sage: SetPartition([[1], [2,3], [4]]).base_set()
{1, 2, 3, 4}
sage: SetPartition([[1,2,3,4]]).base_set()
{1, 2, 3, 4}
sage: SetPartition([]).base_set()
{}
base_set_cardinality()

Return the cardinality of the base set of self, which is the sum of the sizes of the parts of self.

This is also known as the size (sometimes the weight) of a set partition.

EXAMPLES:

sage: SetPartition([[1], [2,3], [4]]).base_set_cardinality()
4
sage: SetPartition([[1,2,3,4]]).base_set_cardinality()
4
coarsenings()

Return a list of coarsenings of self.

See also

refinements()

EXAMPLES:

sage: SetPartition([[1,3],[2,4]]).coarsenings()
[{{1, 2, 3, 4}}, {{1, 3}, {2, 4}}]
sage: SetPartition([[1],[2,4],[3]]).coarsenings()
[{{1, 2, 3, 4}},
 {{1, 2, 4}, {3}},
 {{1, 3}, {2, 4}},
 {{1}, {2, 3, 4}},
 {{1}, {2, 4}, {3}}]
sage: SetPartition([]).coarsenings()
[{}]
inf(other)

The product of the set partitions self and other.

The product of two set partitions \(B\) and \(C\) is defined as the set partition whose parts are the nonempty intersections between each part of \(B\) and each part of \(C\). This product is also the infimum of \(B\) and \(C\) in the classical set partition lattice (that is, the coarsest set partition which is finer than each of \(B\) and \(C\)). Consequently, inf acts as an alias for this method.

See also

sup()

EXAMPLES:

sage: x = SetPartition([ [1,2], [3,5,4] ])
sage: y = SetPartition(( (3,1,2), (5,4) ))
sage: x * y
{{1, 2}, {3}, {4, 5}}

sage: S = SetPartitions(4)
sage: sp1 = S([[2,3,4], [1]])
sage: sp2 = S([[1,3], [2,4]])
sage: s = S([[2,4], [3], [1]])
sage: sp1.inf(sp2) == s
True
max_block_size()

The maximum block size of the diagram.

EXAMPLES:

sage: from sage.combinat.diagram_algebras import PartitionDiagram, PartitionDiagrams
sage: pd = PartitionDiagram([[1,-3,-5],[2,4],[3,-1,-2],[5],[-4]])
sage: pd.max_block_size()
3
sage: sorted(d.max_block_size() for d in PartitionDiagrams(2))
[1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 4]
sage: sorted(sp.max_block_size() for sp in SetPartitions(3))
[1, 2, 2, 2, 3]
standard_form()

Return self as a list of lists.

When the ground set is totally ordered, the elements of each block are listed in increasing order.

This is not related to standard set partitions (which simply means set partitions of \([n] = \{ 1, 2, \ldots , n \}\) for some integer \(n\)) or standardization (standardization()).

EXAMPLES:

sage: [x.standard_form() for x in SetPartitions(4, [2,2])]
[[[1, 2], [3, 4]], [[1, 4], [2, 3]], [[1, 3], [2, 4]]]
sup(t)

Return the supremum of self and t in the classical set partition lattice.

The supremum of two set partitions \(B\) and \(C\) is obtained as the transitive closure of the relation which relates \(i\) to \(j\) if and only if \(i\) and \(j\) are in the same part in at least one of the set partitions \(B\) and \(C\).

See also

__mul__()

EXAMPLES:

sage: S = SetPartitions(4)
sage: sp1 = S([[2,3,4], [1]])
sage: sp2 = S([[1,3], [2,4]])
sage: s = S([[1,2,3,4]])
sage: sp1.sup(sp2) == s
True
class sage.combinat.set_partition.SetPartition(parent, s, check=True)

Bases: sage.combinat.set_partition.AbstractSetPartition

A partition of a set.

A set partition \(p\) of a set \(S\) is a partition of \(S\) into subsets called parts and represented as a set of sets. By extension, a set partition of a nonnegative integer \(n\) is the set partition of the integers from 1 to \(n\). The number of set partitions of \(n\) is called the \(n\)-th Bell number.

There is a natural integer partition associated with a set partition, namely the nonincreasing sequence of sizes of all its parts.

There is a classical lattice associated with all set partitions of \(n\). The infimum of two set partitions is the set partition obtained by intersecting all the parts of both set partitions. The supremum is obtained by transitive closure of the relation \(i\) related to \(j\) if and only if they are in the same part in at least one of the set partitions.

We will use terminology from partitions, in particular the length of a set partition \(A = \{A_1, \ldots, A_k\}\) is the number of parts of \(A\) and is denoted by \(|A| := k\). The size of \(A\) is the cardinality of \(S\). We will also sometimes use the notation \([n] := \{1, 2, \ldots, n\}\).

EXAMPLES:

There are 5 set partitions of the set \(\{1,2,3\}\):

sage: SetPartitions(3).cardinality()
5

Here is the list of them:

sage: SetPartitions(3).list()
[{{1, 2, 3}}, {{1, 2}, {3}}, {{1, 3}, {2}}, {{1}, {2, 3}}, {{1}, {2}, {3}}]

There are 6 set partitions of \(\{1,2,3,4\}\) whose underlying partition is \([2, 1, 1]\):

sage: SetPartitions(4, [2,1,1]).list()
[{{1}, {2, 4}, {3}},
 {{1}, {2}, {3, 4}},
 {{1, 4}, {2}, {3}},
 {{1, 3}, {2}, {4}},
 {{1, 2}, {3}, {4}},
 {{1}, {2, 3}, {4}}]

Since trac ticket #14140, we can create a set partition directly by SetPartition, which creates the base set by taking the union of the parts passed in:

sage: s = SetPartition([[1,3],[2,4]]); s
{{1, 3}, {2, 4}}
sage: s.parent()
Set partitions
apply_permutation(p)

Apply p to the underlying set of self.

INPUT:

  • p – a permutation

EXAMPLES:

sage: x = SetPartition([[1,2], [3,5,4]])
sage: p = Permutation([2,1,4,5,3])
sage: x.apply_permutation(p)
{{1, 2}, {3, 4, 5}}
sage: q = Permutation([3,2,1,5,4])
sage: x.apply_permutation(q)
{{1, 4, 5}, {2, 3}}

sage: m = PerfectMatching([(1,4),(2,6),(3,5)])
sage: m.apply_permutation(Permutation([4,1,5,6,3,2]))
[(1, 2), (3, 5), (4, 6)]
arcs()

Return self as a list of arcs.

Assuming that the blocks are sorted, the arcs are the pairs of consecutive elements in the blocks.

EXAMPLES:

sage: A = SetPartition([[1],[2,3],[4]])
sage: A.arcs()
[(2, 3)]
sage: B = SetPartition([[1,3,6,7],[2,5],[4]])
sage: B.arcs()
[(1, 3), (3, 6), (6, 7), (2, 5)]
cardinality()

Returns the len of self

EXAMPLES:

sage: from sage.structure.list_clone_demo import IncreasingArrays
sage: len(IncreasingArrays()([1,2,3]))
3
check()

Check that we are a valid set partition.

EXAMPLES:

sage: S = SetPartitions(4)
sage: s = S([[1, 3], [2, 4]])
sage: s.check()
closers()

Return the maximal elements of the blocks.

EXAMPLES:

sage: P = SetPartition([[1,2,4,7],[3,9],[5,6,10,11,13],[8],[12]])
sage: P.closers()
[7, 8, 9, 12, 13]
crossings()

Return the crossing arcs of a set partition on a totally ordered set.

OUTPUT:

We place the elements of the ground set in order on a line and draw the set partition by linking consecutive elements of each block in the upper half-plane. This function returns a list of the pairs of crossing lines (as a line correspond to a pair, it returns a list of pairs of pairs).

EXAMPLES:

sage: p = SetPartition([[1,4],[2,5,7],[3,6]])
sage: p.crossings()
[((1, 4), (2, 5)), ((1, 4), (3, 6)), ((2, 5), (3, 6)), ((3, 6), (5, 7))]
crossings_iterator()

Return the crossing arcs of a set partition on a totally ordered set.

OUTPUT:

We place the elements of the ground set in order on a line and draw the set partition by linking consecutive elements of each block in the upper half-plane. This function returns an iterator over the pairs of crossing lines (as a line correspond to a pair, the iterator produces pairs of pairs).

EXAMPLES:

sage: p = SetPartition([[1,4],[2,5,7],[3,6]])
sage: next(p.crossings_iterator())
((1, 4), (2, 5))
is_atomic()

Return if self is an atomic set partition.

A (standard) set partition \(A\) can be split if there exist \(j < i\) such that \(\max(A_j) < \min(A_i)\) where \(A\) is ordered by minimal elements. This means we can write \(A = B | C\) for some nonempty set partitions \(B\) and \(C\). We call a set partition atomic if it cannot be split and is nonempty. Here, the pipe symbol \(|\) is as defined in method pipe().

EXAMPLES:

sage: SetPartition([[1,3], [2]]).is_atomic()
True
sage: SetPartition([[1,3], [2], [4]]).is_atomic()
False
sage: SetPartition([[1], [2,4], [3]]).is_atomic()
False
sage: SetPartition([[1,2,3,4]]).is_atomic()
True
sage: SetPartition([[1, 4], [2], [3]]).is_atomic()
True
sage: SetPartition([]).is_atomic()
False
is_noncrossing()

Check if self is noncrossing.

OUTPUT:

We place the elements of the ground set in order on a line and draw the set partition by linking consecutive elements of each block in the upper half-plane. This function returns True if the picture obtained this way has no crossings.

EXAMPLES:

sage: p = SetPartition([[1,4],[2,5,7],[3,6]])
sage: p.is_noncrossing()
False

sage: n = PerfectMatching([3,8,1,7,6,5,4,2]); n
[(1, 3), (2, 8), (4, 7), (5, 6)]
sage: n.is_noncrossing()
False
sage: PerfectMatching([(1, 4), (2, 3), (5, 6)]).is_noncrossing()
True
is_nonnesting()

Return if self is nonnesting or not.

OUTPUT:

We place the elements of the ground set in order on a line and draw the set partition by linking consecutive elements of each block in the upper half-plane. This function returns True if the picture obtained this way has no nestings.

EXAMPLES:

sage: n = PerfectMatching([3,8,1,7,6,5,4,2]); n
[(1, 3), (2, 8), (4, 7), (5, 6)]
sage: n.is_nonnesting()
False
sage: PerfectMatching([(1, 3), (2, 5), (4, 6)]).is_nonnesting()
True
latex_options()

Return the latex options for use in the _latex_ function as a dictionary. The default values are set using the global options.

Options can be found in set_latex_options()

EXAMPLES:

sage: SP = SetPartition([[1,6], [3,5,4]]); SP.latex_options()
{'angle': 0,
 'color': 'black',
 'fill': False,
 'plot': None,
 'radius': '1cm',
 'show_labels': True,
 'tikz_scale': 1}
nestings()

Return the nestings of self.

OUTPUT:

We place the elements of the ground set in order on a line and draw the set partition by linking consecutive elements of each block in the upper half-plane. This function returns the list of the pairs of nesting lines (as a line correspond to a pair, it returns a list of pairs of pairs).

EXAMPLES:

sage: m = PerfectMatching([(1, 6), (2, 7), (3, 5), (4, 8)])
sage: m.nestings()
[((1, 6), (3, 5)), ((2, 7), (3, 5))]

sage: n = PerfectMatching([3,8,1,7,6,5,4,2]); n
[(1, 3), (2, 8), (4, 7), (5, 6)]
sage: n.nestings()
[((2, 8), (4, 7)), ((2, 8), (5, 6)), ((4, 7), (5, 6))]
nestings_iterator()

Iterate over the nestings of self.

OUTPUT:

We place the elements of the ground set in order on a line and draw the set partition by linking consecutive elements of each block in the upper half-plane. This function returns an iterator over the pairs of nesting lines (as a line correspond to a pair, the iterator produces pairs of pairs).

EXAMPLES:

sage: n = PerfectMatching([(1, 6), (2, 7), (3, 5), (4, 8)])
sage: it = n.nestings_iterator()
sage: next(it)
((1, 6), (3, 5))
sage: next(it)
((2, 7), (3, 5))
sage: next(it)
Traceback (most recent call last):
...
StopIteration
number_of_crossings()

Return the number of crossings.

OUTPUT:

We place the elements of the ground set in order on a line and draw the set partition by linking consecutive elements of each block in the upper half-plane. This function returns the number the pairs of crossing lines.

EXAMPLES:

sage: p = SetPartition([[1,4],[2,5,7],[3,6]])
sage: p.number_of_crossings()
4

sage: n = PerfectMatching([3,8,1,7,6,5,4,2]); n
[(1, 3), (2, 8), (4, 7), (5, 6)]
sage: n.number_of_crossings()
1
number_of_nestings()

Return the number of nestings of self.

OUTPUT:

We place the elements of the ground set in order on a line and draw the set partition by linking consecutive elements of each block in the upper half-plane. This function returns the number the pairs of nesting lines.

EXAMPLES:

sage: n = PerfectMatching([3,8,1,7,6,5,4,2]); n
[(1, 3), (2, 8), (4, 7), (5, 6)]
sage: n.number_of_nestings()
3
openers()

Return the minimal elements of the blocks.

EXAMPLES:

sage: P = SetPartition([[1,2,4,7],[3,9],[5,6,10,11,13],[8],[12]])
sage: P.openers()
[1, 3, 5, 8, 12]
ordered_set_partition_action(s)

Return the action of an ordered set partition s on self.

Let \(A = \{A_1, A_2, \ldots, A_k\}\) be a set partition of some set \(S\) and \(s\) be an ordered set partition (i.e., set composition) of a subset of \([k]\). Let \(A^{\downarrow}\) denote the standardization of \(A\), and \(A_{\{ i_1, i_2, \ldots, i_m \}}\) denote the sub-partition \(\{A_{i_1}, A_{i_2}, \ldots, A_{i_m}\}\) for any subset \(\{i_1, \ldots, i_m\}\) of \(\{1, \ldots, k\}\). We define the set partition \(s(A)\) by

\[s(A) = A_{s_1}^{\downarrow} | A_{s_2}^{\downarrow} | \cdots | A_{s_q}^{\downarrow}.\]

where \(s = (s_1, s_2, \ldots, s_q)\). Here, the pipe symbol \(|\) is as defined in method pipe().

This is \(s[A]\) in section 2.3 in [LM2011].

INPUT:

  • s – an ordered set partition with base set a subset of \(\{1, \ldots, k\}\)

EXAMPLES:

sage: A = SetPartition([[1], [2,4], [3]])
sage: s = OrderedSetPartition([[1,3], [2]])
sage: A.ordered_set_partition_action(s)
{{1}, {2}, {3, 4}}
sage: s = OrderedSetPartition([[2,3], [1]])
sage: A.ordered_set_partition_action(s)
{{1, 3}, {2}, {4}}

We create Figure 1 in [LM2011] (we note that there is a typo in the lower-left corner of the table in the published version of the paper, whereas the arXiv version gives the correct partition):

sage: A = SetPartition([[1,3], [2,9], [4,5,8], [7]])
sage: B = SetPartition([[1,3], [2,8], [4,5,6], [7]])
sage: C = SetPartition([[1,5], [2,8], [3,4,6], [7]])
sage: s = OrderedSetPartition([[1,3], [2]])
sage: t = OrderedSetPartition([[2], [3,4]])
sage: u = OrderedSetPartition([[1], [2,3,4]])
sage: A.ordered_set_partition_action(s)
{{1, 2}, {3, 4, 5}, {6, 7}}
sage: A.ordered_set_partition_action(t)
{{1, 2}, {3, 4, 6}, {5}}
sage: A.ordered_set_partition_action(u)
{{1, 2}, {3, 8}, {4, 5, 7}, {6}}
sage: B.ordered_set_partition_action(s)
{{1, 2}, {3, 4, 5}, {6, 7}}
sage: B.ordered_set_partition_action(t)
{{1, 2}, {3, 4, 5}, {6}}
sage: B.ordered_set_partition_action(u)
{{1, 2}, {3, 8}, {4, 5, 6}, {7}}
sage: C.ordered_set_partition_action(s)
{{1, 4}, {2, 3, 5}, {6, 7}}
sage: C.ordered_set_partition_action(t)
{{1, 2}, {3, 4, 5}, {6}}
sage: C.ordered_set_partition_action(u)
{{1, 2}, {3, 8}, {4, 5, 6}, {7}}

REFERENCES:

pipe(other)

Return the pipe of the set partitions self and other.

The pipe of two set partitions is defined as follows:

For any integer \(k\) and any subset \(I\) of \(\ZZ\), let \(I + k\) denote the subset of \(\ZZ\) obtained by adding \(k\) to every element of \(k\).

If \(B\) and \(C\) are set partitions of \([n]\) and \([m]\), respectively, then the pipe of \(B\) and \(C\) is defined as the set partition

\[\{ B_1, B_2, \ldots, B_b, C_1 + n, C_2 + n, \ldots, C_c + n \}\]

of \([n+m]\), where \(B = \{ B_1, B_2, \ldots, B_b \}\) and \(C = \{ C_1, C_2, \ldots, C_c \}\). This pipe is denoted by \(B | C\).

EXAMPLES:

sage: SetPartition([[1,3],[2,4]]).pipe(SetPartition([[1,3],[2]]))
{{1, 3}, {2, 4}, {5, 7}, {6}}
sage: SetPartition([]).pipe(SetPartition([[1,2],[3,5],[4]]))
{{1, 2}, {3, 5}, {4}}
sage: SetPartition([[1,2],[3,5],[4]]).pipe(SetPartition([]))
{{1, 2}, {3, 5}, {4}}
sage: SetPartition([[1,2],[3]]).pipe(SetPartition([[1]]))
{{1, 2}, {3}, {4}}
plot(angle=None, color='black', base_set_dict=None)

Return a plot of self.

INPUT:

  • angle – (default: \(\pi/4\)) the angle at which the arcs take off (if angle is negative, the arcs are drawn below the horizontal line)
  • color – (default: 'black') color of the arcs
  • base_set_dict – (optional) dictionary with keys elements of base_set() and values as integer or float

EXAMPLES:

sage: p = SetPartition([[1,10,11],[2,3,7],[4,5,6],[8,9]])
sage: p.plot()
Graphics object consisting of 29 graphics primitives
../../_images/set_partition-1.svg
sage: p = SetPartition([[1,3,4],[2,5]])
sage: print(p.plot().description())
Point set defined by 1 point(s):    [(0.0, 0.0)]
Point set defined by 1 point(s):    [(1.0, 0.0)]
Point set defined by 1 point(s):    [(2.0, 0.0)]
Point set defined by 1 point(s):    [(3.0, 0.0)]
Point set defined by 1 point(s):    [(4.0, 0.0)]
Text '1' at the point (0.0,-0.1)
Text '2' at the point (1.0,-0.1)
Text '3' at the point (2.0,-0.1)
Text '4' at the point (3.0,-0.1)
Text '5' at the point (4.0,-0.1)
Arc with center (1.0,-1.0) radii (1.41421356237...,1.41421356237...)
 angle 0.0 inside the sector (0.785398163397...,2.35619449019...)
Arc with center (2.5,-0.5) radii (0.70710678118...,0.70710678118...)
 angle 0.0 inside the sector (0.785398163397...,2.35619449019...)
Arc with center (2.5,-1.5) radii (2.1213203435...,2.1213203435...)
 angle 0.0 inside the sector (0.785398163397...,2.35619449019...)
sage: p = SetPartition([['a','c'],['b','d'],['e']])
sage: print(p.plot().description())
Point set defined by 1 point(s):  [(0.0, 0.0)]
Point set defined by 1 point(s):    [(1.0, 0.0)]
Point set defined by 1 point(s):    [(2.0, 0.0)]
Point set defined by 1 point(s):    [(3.0, 0.0)]
Point set defined by 1 point(s):    [(4.0, 0.0)]
Text 'a' at the point (0.0,-0.1)
Text 'b' at the point (1.0,-0.1)
Text 'c' at the point (2.0,-0.1)
Text 'd' at the point (3.0,-0.1)
Text 'e' at the point (4.0,-0.1)
Arc with center (1.0,-1.0) radii (1.41421356237...,1.41421356237...)
 angle 0.0 inside the sector (0.785398163397...,2.35619449019...)
Arc with center (2.0,-1.0) radii (1.41421356237...,1.41421356237...)
 angle 0.0 inside the sector (0.785398163397...,2.35619449019...)
sage: p = SetPartition([['a','c'],['b','d'],['e']])
sage: print(p.plot(base_set_dict={'a':0,'b':1,'c':2,'d':-2.3,'e':5.4}).description())
Point set defined by 1 point(s):    [(-2.3, 0.0)]
Point set defined by 1 point(s):    [(0.0, 0.0)]
Point set defined by 1 point(s):    [(1.0, 0.0)]
Point set defined by 1 point(s):    [(2.0, 0.0)]
Point set defined by 1 point(s):    [(5.4, 0.0)]
Text 'a' at the point (0.0,-0.1)
Text 'b' at the point (1.0,-0.1)
Text 'c' at the point (2.0,-0.1)
Text 'd' at the point (-2.3,-0.1)
Text 'e' at the point (5.4,-0.1)
Arc with center (-0.6...,-1.65) radii (2.3334523779...,2.3334523779...)
 angle 0.0 inside the sector (0.785398163397...,2.35619449019...)
Arc with center (1.0,-1.0) radii (1.4142135623...,1.4142135623...)
 angle 0.0 inside the sector (0.785398163397...,2.35619449019...)
refinements()

Return a list of refinements of self.

See also

coarsenings()

EXAMPLES:

sage: SetPartition([[1,3],[2,4]]).refinements()
[{{1, 3}, {2, 4}},
 {{1, 3}, {2}, {4}},
 {{1}, {2, 4}, {3}},
 {{1}, {2}, {3}, {4}}]
sage: SetPartition([[1],[2,4],[3]]).refinements()
[{{1}, {2, 4}, {3}}, {{1}, {2}, {3}, {4}}]
sage: SetPartition([]).refinements()
[{}]
restriction(I)

Return the restriction of self to a subset I (which is given as a set or list or any other iterable).

EXAMPLES:

sage: A = SetPartition([[1], [2,3]])
sage: A.restriction([1,2])
{{1}, {2}}
sage: A.restriction([2,3])
{{2, 3}}
sage: A.restriction([])
{}
sage: A.restriction([4])
{}
set_latex_options(**kwargs)

Set the latex options for use in the _latex_ function

  • tikz_scale – (default: 1) scale for use with tikz package
  • plot – (default: None) None returns the set notation, linear returns a linear plot, cyclic returns a cyclic plot
  • color – (default: 'black') the arc colors
  • fill – (default: False) if True then fills color, else you can pass in a color to alter the fill color - only works with cyclic plot
  • show_labels – (default: True) if True shows labels - only works with plots
  • radius – (default: "1cm") radius of circle for cyclic plot - only works with cyclic plot
  • angle – (default: 0) angle for linear plot

EXAMPLES:

sage: SP = SetPartition([[1,6], [3,5,4]])
sage: SP.set_latex_options(tikz_scale=2,plot='linear',fill=True,color='blue',angle=45)
sage: SP.set_latex_options(plot='cyclic')
sage: SP.latex_options()
{'angle': 45,
 'color': 'blue',
 'fill': True,
 'plot': 'cyclic',
 'radius': '1cm',
 'show_labels': True,
 'tikz_scale': 2}
shape()

Return the integer partition whose parts are the sizes of the sets in self.

EXAMPLES:

sage: S = SetPartitions(5)
sage: x = S([[1,2], [3,5,4]])
sage: x.shape()
[3, 2]
sage: y = S([[2], [3,1], [5,4]])
sage: y.shape()
[2, 2, 1]
shape_partition()

Return the integer partition whose parts are the sizes of the sets in self.

EXAMPLES:

sage: S = SetPartitions(5)
sage: x = S([[1,2], [3,5,4]])
sage: x.shape()
[3, 2]
sage: y = S([[2], [3,1], [5,4]])
sage: y.shape()
[2, 2, 1]
size()

Return the cardinality of the base set of self, which is the sum of the sizes of the parts of self.

This is also known as the size (sometimes the weight) of a set partition.

EXAMPLES:

sage: SetPartition([[1], [2,3], [4]]).base_set_cardinality()
4
sage: SetPartition([[1,2,3,4]]).base_set_cardinality()
4
standardization()

Return the standardization of self.

Given a set partition \(A = \{A_1, \ldots, A_n\}\) of an ordered set \(S\), the standardization of \(A\) is the set partition of \(\{1, 2, \ldots, |S|\}\) obtained by replacing the elements of the parts of \(A\) by the integers \(1, 2, \ldots, |S|\) in such a way that their relative order is preserved (i. e., the smallest element in the whole set partition is replaced by \(1\), the next-smallest by \(2\), and so on).

EXAMPLES:

sage: SetPartition([[4], [1, 3]]).standardization()
{{1, 2}, {3}}
sage: SetPartition([[4], [6, 3]]).standardization()
{{1, 3}, {2}}
sage: SetPartition([]).standardization()
{}
sage: SetPartition([('c','b'),('d','f'),('e','a')]).standardization()
{{1, 5}, {2, 3}, {4, 6}}
strict_coarsenings()

Return all strict coarsenings of self.

Strict coarsening is the binary relation on set partitions defined as the transitive-and-reflexive closure of the relation \(\prec\) defined as follows: For two set partitions \(A\) and \(B\), we have \(A \prec B\) if there exist parts \(A_i, A_j\) of \(A\) such that \(\max(A_i) < \min(A_j)\) and \(B = A \setminus \{A_i, A_j\} \cup \{ A_i \cup A_j \}\).

EXAMPLES:

sage: A = SetPartition([[1],[2,3],[4]])
sage: A.strict_coarsenings()
[{{1}, {2, 3}, {4}}, {{1, 2, 3}, {4}}, {{1, 4}, {2, 3}},
 {{1}, {2, 3, 4}}, {{1, 2, 3, 4}}]
sage: SetPartition([[1],[2,4],[3]]).strict_coarsenings()
[{{1}, {2, 4}, {3}}, {{1, 2, 4}, {3}}, {{1, 3}, {2, 4}}]
sage: SetPartition([]).strict_coarsenings()
[{}]
to_partition()

Return the integer partition whose parts are the sizes of the sets in self.

EXAMPLES:

sage: S = SetPartitions(5)
sage: x = S([[1,2], [3,5,4]])
sage: x.shape()
[3, 2]
sage: y = S([[2], [3,1], [5,4]])
sage: y.shape()
[2, 2, 1]
to_permutation()

Convert a set partition of \(\{1,...,n\}\) to a permutation by considering the blocks of the partition as cycles.

The cycles are such that the number of excedences is maximised, that is, each cycle is of the form \((a_1,a_2, ...,a_k)\) with \(a_1<a_2<...<a_k\).

EXAMPLES:

sage: s = SetPartition([[1,3],[2,4]])
sage: s.to_permutation()
[3, 4, 1, 2]
to_restricted_growth_word(bijection='blocks')

Convert a set partition of \(\{1,...,n\}\) to a word of length \(n\) with letters in the non-negative integers such that each letter is at most 1 larger than all the letters before.

INPUT:

OUTPUT:

A restricted growth word.

EXAMPLES:

sage: P = SetPartition([[1,4],[2,8],[3,5,6,9],[7]])
sage: P.to_restricted_growth_word()
[0, 1, 2, 0, 2, 2, 3, 1, 2]

sage: P.to_restricted_growth_word("intertwining")
[0, 1, 2, 2, 1, 0, 3, 3, 2]

sage: P = SetPartition([[1,2,4,7],[3,9],[5,6,10,11,13],[8],[12]])
sage: P.to_restricted_growth_word()
[0, 0, 1, 0, 2, 2, 0, 3, 1, 2, 2, 4, 2]

sage: P.to_restricted_growth_word("intertwining")
[0, 0, 1, 1, 2, 0, 1, 3, 3, 3, 0, 4, 1]
to_restricted_growth_word_blocks()

Convert a set partition of \(\{1,...,n\}\) to a word of length \(n\) with letters in the non-negative integers such that each letter is at most 1 larger than all the letters before.

The word is obtained by sorting the blocks by their minimal element and setting the letters at the positions of the elements in the \(i\)-th block to \(i\).

OUTPUT:

  • a restricted growth word.

EXAMPLES:

sage: P = SetPartition([[1,4],[2,8],[3,5,6,9],[7]])
sage: P.to_restricted_growth_word_blocks()
[0, 1, 2, 0, 2, 2, 3, 1, 2]
to_restricted_growth_word_intertwining()

Convert a set partition of \(\{1,...,n\}\) to a word of length \(n\) with letters in the non-negative integers such that each letter is at most 1 larger than all the letters before.

The \(i\)-th letter of the word is the numbers of crossings of the arc (or half-arc) in the extended arc diagram ending at \(i\), with arcs (or half-arcs) beginning at a smaller element and ending at a larger element.

OUTPUT:

  • a restricted growth word.

EXAMPLES:

sage: P = SetPartition([[1,4],[2,8],[3,5,6,9],[7]])
sage: P.to_restricted_growth_word_intertwining()
[0, 1, 2, 2, 1, 0, 3, 3, 2]
to_rook_placement(bijection='arcs')

Return a set of pairs defining a placement of non-attacking rooks on a triangular board.

The cells of the board corresponding to a set partition of \(\{1,...,n\}\) are the pairs \((i,j)\) with \(0 < i < j < n+1\).

INPUT:

EXAMPLES:

sage: P = SetPartition([[1,2,4,7],[3,9],[5,6,10,11,13],[8],[12]])
sage: P.to_rook_placement()
[(1, 2), (2, 4), (4, 7), (3, 9), (5, 6), (6, 10), (10, 11), (11, 13)]
sage: P.to_rook_placement("gamma")
[(1, 4), (3, 5), (4, 6), (5, 8), (7, 11), (8, 9), (10, 12), (12, 13)]
sage: P.to_rook_placement("rho")
[(1, 2), (2, 6), (3, 4), (4, 10), (5, 9), (6, 7), (10, 11), (11, 13)]
sage: P.to_rook_placement("psi")
[(1, 2), (2, 6), (3, 4), (5, 9), (6, 7), (7, 10), (9, 11), (11, 13)]
to_rook_placement_gamma()

Return the rook diagram obtained by placing rooks according to Wachs and White’s bijection gamma.

Note that our index convention differs from the convention in [WW1991]: regarding the rook board as a lower-right triangular grid, we refer with \((i,j)\) to the cell in the \(i\)-th column from the right and the \(j\)-th row from the top.

The algorithm proceeds as follows: non-attacking rooks are placed beginning at the left column. If \(n+1-i\) is an opener, column \(i\) remains empty. Otherwise, we place a rook into column \(i\), such that the number of cells below the rook, which are not yet attacked by another rook, equals the index of the block to which \(n+1-i\) belongs.

OUTPUT:

A list of coordinates.

EXAMPLES:

sage: P = SetPartition([[1,4],[2,8],[3,5,6,9],[7]])
sage: P.to_rook_placement_gamma()
[(1, 3), (2, 7), (4, 5), (5, 6), (6, 9)]

Figure 5 in [WW1991]:

sage: P = SetPartition([[1,2,4,7],[3,9],[5,6,10,11,13],[8],[12]])
sage: r = P.to_rook_placement_gamma(); r
[(1, 4), (3, 5), (4, 6), (5, 8), (7, 11), (8, 9), (10, 12), (12, 13)]
to_rook_placement_psi()

Return the rook diagram obtained by placing rooks according to Yip’s bijection psi.

OUTPUT:

A list of coordinates.

EXAMPLES:

Example 36 (arXiv version: Example 4.5) in [Yip2018]:

sage: P = SetPartition([[1, 5], [2], [3, 8, 9], [4], [6, 7]])
sage: P.to_rook_placement_psi()
[(1, 7), (3, 8), (4, 5), (7, 9)]

Note that the columns corresponding to the minimal elements of the blocks remain empty.

to_rook_placement_rho()

Return the rook diagram obtained by placing rooks according to Wachs and White’s bijection rho.

Note that our index convention differs from the convention in [WW1991]: regarding the rook board as a lower-right triangular grid, we refer with \((i,j)\) to the cell in the \(i\)-th column from the right and the \(j\)-th row from the top.

The algorithm proceeds as follows: non-attacking rooks are placed beginning at the top row. The columns corresponding to the closers of the set partition remain empty. Let \(rs_j\) be the the number of closers which are larger than \(j\) and whose block is before the block of \(j\).

We then place a rook into row \(j\), such that the number of cells to the left of the rook, which are not yet attacked by another rook and are not in a column corresponding to a closer, equals \(rs_j\), unless there are not enough cells in this row available, in which case the row remains empty.

One can show that the precisely those rows which correspond to openers of the set partition remain empty.

OUTPUT:

A list of coordinates.

EXAMPLES:

sage: P = SetPartition([[1,4],[2,8],[3,5,6,9],[7]])
sage: P.to_rook_placement_rho()
[(1, 5), (2, 6), (3, 4), (5, 9), (6, 8)]

Figure 6 in [WW1991]:

sage: P = SetPartition([[1,2,4,7],[3,9],[5,6,10,11,13],[8],[12]])
sage: r = P.to_rook_placement_rho(); r
[(1, 2), (2, 6), (3, 4), (4, 10), (5, 9), (6, 7), (10, 11), (11, 13)]

sage: sorted(P.closers() + [i for i, _ in r]) == list(range(1,14))
True
sage: sorted(P.openers() + [j for _, j in r]) == list(range(1,14))
True
class sage.combinat.set_partition.SetPartitions

Bases: sage.structure.unique_representation.UniqueRepresentation, sage.structure.parent.Parent

An (unordered) partition of a set \(S\) is a set of pairwise disjoint nonempty subsets with union \(S\), and is represented by a sorted list of such subsets.

SetPartitions(s) returns the class of all set partitions of the set s, which can be given as a set or a string; if a string, each character is considered an element.

SetPartitions(n), where n is an integer, returns the class of all set partitions of the set \(\{1, 2, \ldots, n\}\).

You may specify a second argument \(k\). If \(k\) is an integer, SetPartitions returns the class of set partitions into \(k\) parts; if it is an integer partition, SetPartitions returns the class of set partitions whose block sizes correspond to that integer partition.

The Bell number \(B_n\), named in honor of Eric Temple Bell, is the number of different partitions of a set with \(n\) elements.

EXAMPLES:

sage: S = [1,2,3,4]
sage: SetPartitions(S, 2)
Set partitions of {1, 2, 3, 4} with 2 parts
sage: SetPartitions([1,2,3,4], [3,1]).list()
[{{1}, {2, 3, 4}}, {{1, 2, 3}, {4}}, {{1, 2, 4}, {3}}, {{1, 3, 4}, {2}}]
sage: SetPartitions(7, [3,3,1]).cardinality()
70

In strings, repeated letters are not considered distinct as of trac ticket #14140:

sage: SetPartitions('abcde').cardinality()
52
sage: SetPartitions('aabcd').cardinality()
15

REFERENCES:

Element

alias of SetPartition

from_arcs(arcs, n)

Return the coarsest set partition of \(\{1,...,n\}\) such that any two elements connected by an arc are in the same block.

INPUT:

  • n – an integer specifying the size of the set partition to be produced.
  • arcs – a list of pairs specifying which elements are in the same block.

EXAMPLES:

sage: SetPartitions().from_arcs([(2,3)], 5)
{{1}, {2, 3}, {4}, {5}}
from_restricted_growth_word(w, bijection='blocks')

Convert a word of length \(n\) with letters in the non-negative integers such that each letter is at most 1 larger than all the letters before to a set partition of \(\{1,...,n\}\).

INPUT:

  • w – a restricted growth word.
  • bijection (default: blocks) – defines the map from restricted growth functions to set partitions. These are currently:

OUTPUT:

A set partition.

EXAMPLES:

sage: SetPartitions().from_restricted_growth_word([0, 1, 2, 0, 2, 2, 3, 1, 2])
{{1, 4}, {2, 8}, {3, 5, 6, 9}, {7}}

sage: SetPartitions().from_restricted_growth_word([0, 0, 1, 0, 2, 2, 0, 3, 1, 2, 2, 4, 2])
{{1, 2, 4, 7}, {3, 9}, {5, 6, 10, 11, 13}, {8}, {12}}

sage: SetPartitions().from_restricted_growth_word([0, 0, 1, 0, 2, 2, 0, 3, 1, 2, 2, 4, 2], "intertwining")
{{1, 2, 6, 7, 9}, {3, 4}, {5, 10, 13}, {8, 11}, {12}}
from_restricted_growth_word_blocks(w)

Convert a word of length \(n\) with letters in the non-negative integers such that each letter is at most 1 larger than all the letters before to a set partition of \(\{1,...,n\}\).

w[i]` is the index of the block containing ``i+1 when sorting the blocks by their minimal element.

INPUT:

  • w – a restricted growth word.

OUTPUT:

A set partition.

EXAMPLES:

sage: SetPartitions().from_restricted_growth_word_blocks([0, 0, 1, 0, 2, 2, 0, 3, 1, 2, 2, 4, 2])
{{1, 2, 4, 7}, {3, 9}, {5, 6, 10, 11, 13}, {8}, {12}}
from_restricted_growth_word_intertwining(w)

Convert a word of length \(n\) with letters in the non-negative integers such that each letter is at most 1 larger than all the letters before to a set partition of \(\{1,...,n\}\).

The \(i\)-th letter of the word is the numbers of crossings of the arc (or half-arc) in the extended arc diagram ending at \(i\), with arcs (or half-arcs) beginning at a smaller element and ending at a larger element.

INPUT:

  • w – a restricted growth word.

OUTPUT:

A set partition.

EXAMPLES:

sage: SetPartitions().from_restricted_growth_word_intertwining([0, 0, 1, 0, 2, 2, 0, 3, 1, 2, 2, 4, 2])
{{1, 2, 6, 7, 9}, {3, 4}, {5, 10, 13}, {8, 11}, {12}}
from_rook_placement(rooks, bijection='arcs', n=None)

Convert a rook placement of the triangular grid to a set partition of \(\{1,...,n\}\).

If n is not given, it is first checked whether it can be determined from the parent, otherwise it is the maximal occuring integer in the set of rooks.

INPUT:

EXAMPLES:

sage: SetPartitions(9).from_rook_placement([[1,4],[2,8],[3,5],[5,6],[6,9]])
{{1, 4}, {2, 8}, {3, 5, 6, 9}, {7}}

sage: SetPartitions(13).from_rook_placement([[12,13],[10,12],[8,9],[7,11],[5,8],[4,6],[3,5],[1,4]], "gamma")
{{1, 2, 4, 7}, {3, 9}, {5, 6, 10, 11, 13}, {8}, {12}}
from_rook_placement_gamma(rooks, n)

Return the set partition of \(\{1,...,n\}\) corresponding to the given rook placement by applying Wachs and White’s bijection gamma.

Note that our index convention differs from the convention in [WW1991]: regarding the rook board as a lower-right triangular grid, we refer with \((i,j)\) to the cell in the \(i\)-th column from the right and the \(j\)-th row from the top.

INPUT:

  • n – an integer specifying the size of the set partition to be produced.
  • rooks – a list of pairs \((i,j)\) such that \(0 < i < j < n+1\).

OUTPUT:

A set partition.

EXAMPLES:

Figure 5 in [WW1991] concerns the following rook placement:

sage: r = [(1, 4), (3, 5), (4, 6), (5, 8), (7, 11), (8, 9), (10, 12), (12, 13)]

Note that the rook \((1, 4)\), translated into Wachs and White’s convention, is a rook in row 4 from the top and column 13 from the left. The corresponding set partition is:

sage: SetPartitions().from_rook_placement_gamma(r, 13)
{{1, 2, 4, 7}, {3, 9}, {5, 6, 10, 11, 13}, {8}, {12}}
from_rook_placement_psi(rooks, n)

Return the set partition of \(\{1,...,n\}\) corresponding to the given rook placement by applying Yip’s bijection psi.

INPUT:

  • n – an integer specifying the size of the set partition to be produced.
  • rooks – a list of pairs \((i,j)\) such that \(0 < i < j < n+1\).

OUTPUT:

A set partition.

EXAMPLES:

Example 36 (arXiv version: Example 4.5) in [Yip2018] concerns the following rook placement:

sage: r = [(4,5), (1,7), (3, 8), (7,9)]
sage: SetPartitions().from_rook_placement_psi(r, 9)
{{1, 5}, {2}, {3, 8, 9}, {4}, {6, 7}}
from_rook_placement_rho(rooks, n)

Return the set partition of \(\{1,...,n\}\) corresponding to the given rook placement by applying Wachs and White’s bijection rho.

Note that our index convention differs from the convention in [WW1991]: regarding the rook board as a lower-right triangular grid, we refer with \((i,j)\) to the cell in the \(i\)-th column from the right and the \(j\)-th row from the top.

INPUT:

  • n – an integer specifying the size of the set partition to be produced.
  • rooks – a list of pairs \((i,j)\) such that \(0 < i < j < n+1\).

OUTPUT:

A set partition.

EXAMPLES:

Figure 5 in [WW1991] concerns the following rook placement:

sage: r = [(1, 2), (2, 6), (3, 4), (4, 10), (5, 9), (6, 7), (10, 11), (11, 13)]

Note that the rook \((1, 2)\), translated into Wachs and White’s convention, is a rook in row 2 from the top and column 13 from the left. The corresponding set partition is:

sage: SetPartitions().from_rook_placement_rho(r, 13)
{{1, 2, 4, 7}, {3, 9}, {5, 6, 10, 11, 13}, {8}, {12}}
is_less_than(s, t)

Check if \(s < t\) in the refinement ordering on set partitions.

This means that \(s\) is a refinement of \(t\) and satisfies \(s \neq t\).

A set partition \(s\) is said to be a refinement of a set partition \(t\) of the same set if and only if each part of \(s\) is a subset of a part of \(t\).

EXAMPLES:

sage: S = SetPartitions(4)
sage: s = S([[1,3],[2,4]])
sage: t = S([[1],[2],[3],[4]])
sage: S.is_less_than(t, s)
True
sage: S.is_less_than(s, t)
False
sage: S.is_less_than(s, s)
False
is_strict_refinement(s, t)

Return True if s is a strict refinement of t and satisfies \(s \neq t\).

A set partition \(s\) is said to be a strict refinement of a set partition \(t\) of the same set if and only if one can obtain \(t\) from \(s\) by repeatedly combining pairs of parts whose convex hulls don’t intersect (i. e., whenever we are combining two parts, the maximum of each of them should be smaller than the minimum of the other).

EXAMPLES:

sage: S = SetPartitions(4)
sage: s = S([[1],[2],[3],[4]])
sage: t = S([[1,3],[2,4]])
sage: u = S([[1,2,3,4]])
sage: S.is_strict_refinement(s, t)
True
sage: S.is_strict_refinement(t, u)
False
sage: A = SetPartition([[1,3],[2,4]])
sage: B = SetPartition([[1,2,3,4]])
sage: S.is_strict_refinement(s, A)
True
sage: S.is_strict_refinement(t, B)
False
lt(s, t)

Check if \(s < t\) in the refinement ordering on set partitions.

This means that \(s\) is a refinement of \(t\) and satisfies \(s \neq t\).

A set partition \(s\) is said to be a refinement of a set partition \(t\) of the same set if and only if each part of \(s\) is a subset of a part of \(t\).

EXAMPLES:

sage: S = SetPartitions(4)
sage: s = S([[1,3],[2,4]])
sage: t = S([[1],[2],[3],[4]])
sage: S.is_less_than(t, s)
True
sage: S.is_less_than(s, t)
False
sage: S.is_less_than(s, s)
False
class sage.combinat.set_partition.SetPartitions_all

Bases: sage.combinat.set_partition.SetPartitions

All set partitions.

class sage.combinat.set_partition.SetPartitions_set(s)

Bases: sage.combinat.set_partition.SetPartitions

Set partitions of a fixed set \(S\).

base_set()

Return the base set of self.

EXAMPLES:

sage: SetPartitions(3).base_set()
{1, 2, 3}

sage: sorted(SetPartitions(["a", "b", "c"]).base_set())
['a', 'b', 'c']
base_set_cardinality()

Return the cardinality of the base set of self.

EXAMPLES:

sage: SetPartitions(3).base_set_cardinality()
3
cardinality()

Return the number of set partitions of the set \(S\).

The cardinality is given by the \(n\)-th Bell number where \(n\) is the number of elements in the set \(S\).

EXAMPLES:

sage: SetPartitions([1,2,3,4]).cardinality()
15
sage: SetPartitions(3).cardinality()
5
sage: SetPartitions(3,2).cardinality()
3
sage: SetPartitions([]).cardinality()
1
random_element()

Return a random set partition.

This is a very naive implementation of Knuths outline in F3B, 7.2.1.5.

EXAMPLES:

sage: S = SetPartitions(10)
sage: S.random_element()
{{1, 4, 9}, {2, 5, 7}, {3}, {6}, {8, 10}}

sage: S = SetPartitions(["a", "b", "c"])
sage: S.random_element() # random
{{'a'}, {'b', 'c'}}
class sage.combinat.set_partition.SetPartitions_setn(s, k)

Bases: sage.combinat.set_partition.SetPartitions_set

Set partitions with a given number of blocks.

cardinality()

The Stirling number of the second kind is the number of partitions of a set of size \(n\) into \(k\) blocks.

EXAMPLES:

sage: SetPartitions(5, 3).cardinality()
25
sage: stirling_number2(5,3)
25
n

self.n is deprecated; use number_of_blocks() instead.

number_of_blocks()

Return the number of blocks of the set partitions in self.

EXAMPLES:

sage: SetPartitions(5, 3).number_of_blocks()
3
random_element()

Return a random set partition of self.

See https://mathoverflow.net/questions/141999.

EXAMPLES:

sage: S = SetPartitions(10, 4)
sage: S.random_element()
{{1, 2, 4, 6, 9, 10}, {3}, {5, 7}, {8}}

sage: SetPartitions(["a", "b", "c"], 2).random_element() # random
{{'a'}, {'b', 'c'}}
class sage.combinat.set_partition.SetPartitions_setparts(s, parts)

Bases: sage.combinat.set_partition.SetPartitions_set

Set partitions with fixed partition sizes corresponding to an integer partition \(\lambda\).

cardinality()

Return the cardinality of self.

This algorithm counts for each block of the partition the number of ways to fill it using values from the set. Then, for each distinct value \(v\) of block size, we divide the result by the number of ways to arrange the blocks of size \(v\) in the set partition.

For example, if we want to count the number of set partitions of size 13 having [3,3,3,2,2] as underlying partition we compute the number of ways to fill each block of the partition, which is \(\binom{13}{3} \binom{10}{3} \binom{7}{3} \binom{4}{2}\binom{2}{2}\) and as we have three blocks of size \(3\) and two blocks of size \(2\), we divide the result by \(3!2!\) which gives us \(600600\).

EXAMPLES:

sage: SetPartitions(3, [2,1]).cardinality()
3
sage: SetPartitions(13, Partition([3,3,3,2,2])).cardinality()
600600
parts

self.parts is deprecated; use shape() instead.

shape()

Return the partition of block sizes of the set partitions in self.

EXAMPLES:

sage: SetPartitions(5, [2,2,1]).shape()
[2, 2, 1]
sage.combinat.set_partition.cyclic_permutations_of_set_partition(set_part)

Returns all combinations of cyclic permutations of each cell of the set partition.

AUTHORS:

  • Robert L. Miller

EXAMPLES:

sage: from sage.combinat.set_partition import cyclic_permutations_of_set_partition
sage: cyclic_permutations_of_set_partition([[1,2,3,4],[5,6,7]])
[[[1, 2, 3, 4], [5, 6, 7]],
 [[1, 2, 4, 3], [5, 6, 7]],
 [[1, 3, 2, 4], [5, 6, 7]],
 [[1, 3, 4, 2], [5, 6, 7]],
 [[1, 4, 2, 3], [5, 6, 7]],
 [[1, 4, 3, 2], [5, 6, 7]],
 [[1, 2, 3, 4], [5, 7, 6]],
 [[1, 2, 4, 3], [5, 7, 6]],
 [[1, 3, 2, 4], [5, 7, 6]],
 [[1, 3, 4, 2], [5, 7, 6]],
 [[1, 4, 2, 3], [5, 7, 6]],
 [[1, 4, 3, 2], [5, 7, 6]]]
sage.combinat.set_partition.cyclic_permutations_of_set_partition_iterator(set_part)

Iterates over all combinations of cyclic permutations of each cell of the set partition.

AUTHORS:

  • Robert L. Miller

EXAMPLES:

sage: from sage.combinat.set_partition import cyclic_permutations_of_set_partition_iterator
sage: list(cyclic_permutations_of_set_partition_iterator([[1,2,3,4],[5,6,7]]))
[[[1, 2, 3, 4], [5, 6, 7]],
 [[1, 2, 4, 3], [5, 6, 7]],
 [[1, 3, 2, 4], [5, 6, 7]],
 [[1, 3, 4, 2], [5, 6, 7]],
 [[1, 4, 2, 3], [5, 6, 7]],
 [[1, 4, 3, 2], [5, 6, 7]],
 [[1, 2, 3, 4], [5, 7, 6]],
 [[1, 2, 4, 3], [5, 7, 6]],
 [[1, 3, 2, 4], [5, 7, 6]],
 [[1, 3, 4, 2], [5, 7, 6]],
 [[1, 4, 2, 3], [5, 7, 6]],
 [[1, 4, 3, 2], [5, 7, 6]]]