# Group, ring, etc. actions on objects¶

The terminology and notation used is suggestive of groups acting on sets, but this framework can be used for modules, algebras, etc.

A group action $$G \times S \rightarrow S$$ is a functor from $$G$$ to Sets.

Warning

An Action object only keeps a weak reference to the underlying set which is acted upon. This decision was made in trac ticket #715 in order to allow garbage collection within the coercion framework (this is where actions are mainly used) and avoid memory leaks.

sage: from sage.categories.action import Action
sage: class P: pass
sage: A = Action(P(),P())
sage: import gc
sage: _ = gc.collect()
sage: A
<repr(<sage.categories.action.Action at 0x...>) failed: RuntimeError: This action acted on a set that became garbage collected>

To avoid garbage collection of the underlying set, it is sufficient to create a strong reference to it before the action is created.

sage: _ = gc.collect()
sage: from sage.categories.action import Action
sage: class P: pass
sage: q = P()
sage: A = Action(P(),q)
sage: gc.collect()
0
sage: A
Left action by <__main__.P ... at ...> on <__main__.P ... at ...>

AUTHOR:

• Robert Bradshaw: initial version
class sage.categories.action.Action

The action of G on S.

INPUT:

• G – a parent or Python type
• S – a parent or Python type
• is_left – (boolean, default: True) whether elements of G are on the left
• op – (default: None) operation. This is not used by Action itself, but other classes may use it
G
act(g, x)

This is a consistent interface for acting on x by g, regardless of whether it’s a left or right action.

If needed, g and x are converted to the correct parent.

EXAMPLES:

sage: R.<x> = ZZ []
sage: from sage.structure.coerce_actions import IntegerMulAction
sage: A = IntegerMulAction(ZZ, R, True)   # Left action
sage: A.act(5, x)
5*x
sage: A.act(int(5), x)
5*x
sage: A = IntegerMulAction(ZZ, R, False)  # Right action
sage: A.act(5, x)
5*x
sage: A.act(int(5), x)
5*x
actor()
codomain()
domain()
is_left()
left_domain()
op
operation()
right_domain()
class sage.categories.action.ActionEndomorphism

The endomorphism defined by the action of one element.

EXAMPLES:

sage: A = ZZ['x'].get_action(QQ, self_on_left=False, op=operator.mul)
sage: A
Left scalar multiplication by Rational Field on Univariate Polynomial
Ring in x over Integer Ring
sage: A(1/2)
Action of 1/2 on Univariate Polynomial Ring in x over Integer Ring
under Left scalar multiplication by Rational Field on Univariate
Polynomial Ring in x over Integer Ring.
class sage.categories.action.InverseAction

An action that acts as the inverse of the given action.

EXAMPLES:

sage: V = QQ^3
sage: v = V((1, 2, 3))
sage: cm = get_coercion_model()

sage: a = cm.get_action(V, QQ, operator.mul)
sage: a
Right scalar multiplication by Rational Field on Vector space of dimension 3 over Rational Field
sage: ~a
Right inverse action by Rational Field on Vector space of dimension 3 over Rational Field
sage: (~a)(v, 1/3)
(3, 6, 9)

sage: b = cm.get_action(QQ, V, operator.mul)
sage: b
Left scalar multiplication by Rational Field on Vector space of dimension 3 over Rational Field
sage: ~b
Left inverse action by Rational Field on Vector space of dimension 3 over Rational Field
sage: (~b)(1/3, v)
(3, 6, 9)

sage: c = cm.get_action(ZZ, list, operator.mul)
sage: c
Left action by Integer Ring on <... 'list'>
sage: ~c
Traceback (most recent call last):
...
TypeError: no inverse defined for Left action by Integer Ring on <... 'list'>
codomain()
class sage.categories.action.PrecomposedAction

A precomposed action first applies given maps, and then applying an action to the return values of the maps.

EXAMPLES:

We demonstrate that an example discussed on trac ticket #14711 did not become a problem:

sage: E = ModularSymbols(11).2
sage: s = E.modular_symbol_rep()
sage: del E,s
sage: import gc
sage: _ = gc.collect()
sage: E = ModularSymbols(11).2
sage: v = E.manin_symbol_rep()
sage: c,x = v[0]
sage: y = x.modular_symbol_rep()
sage: coercion_model.get_action(QQ, parent(y), op=operator.mul)
Left scalar multiplication by Rational Field on Abelian Group of all Formal Finite Sums over Rational Field
with precomposition on right by Coercion map:
From: Abelian Group of all Formal Finite Sums over Integer Ring
To:   Abelian Group of all Formal Finite Sums over Rational Field
codomain()
domain()
left_precomposition

The left map to precompose with, or None if there is no left precomposition map.

right_precomposition

The right map to precompose with, or None if there is no right precomposition map.