# The Real Line and Open Intervals¶

The class OpenInterval implement open intervals as 1-dimensional differentiable manifolds over $$\RR$$. The derived class RealLine is devoted to $$\RR$$ itself, as the open interval $$(-\infty, +\infty)$$.

AUTHORS:

• Eric Gourgoulhon (2015): initial version
• Travis Scrimshaw (2016): review tweaks

REFERENCES:

class sage.manifolds.differentiable.real_line.OpenInterval(lower, upper, ambient_interval=None, name=None, latex_name=None, coordinate=None, names=None, start_index=0)

Open interval as a 1-dimensional differentiable manifold over $$\RR$$.

INPUT:

• lower – lower bound of the interval (possibly -Infinity)
• upper – upper bound of the interval (possibly +Infinity)
• ambient_interval – (default: None) another open interval, to which the constructed interval is a subset of
• name – (default: None) string; name (symbol) given to the interval; if None, the name is constructed from lower and upper
• latex_name – (default: None) string; LaTeX symbol to denote the interval; if None, the LaTeX symbol is constructed from lower and upper if name is None, otherwise, it is set to name
• coordinate – (default: None) string defining the symbol of the canonical coordinate set on the interval; if none is provided and names is None, the symbol ‘t’ is used
• names – (default: None) used only when coordinate is None: it must be a single-element tuple containing the canonical coordinate symbol (this is guaranteed if the shortcut <names> is used, see examples below)
• start_index – (default: 0) unique value of the index for vectors and forms on the interval manifold

EXAMPLES:

The interval $$(0,\pi)$$:

sage: I = OpenInterval(0, pi); I
Real interval (0, pi)
sage: latex(I)
\left(0, \pi\right)


I is a 1-dimensional smooth manifold over $$\RR$$:

sage: I.category()
Category of smooth manifolds over Real Field with 53 bits of precision
sage: I.base_field()
Real Field with 53 bits of precision
sage: dim(I)
1


It is infinitely differentiable (smooth manifold):

sage: I.diff_degree()
+Infinity


The instance is unique (as long as the constructor arguments are the same):

sage: I is OpenInterval(0, pi)
True
sage: I is OpenInterval(0, pi, name='I')
False


The display of the interval can be customized:

sage: I  # default display
Real interval (0, pi)
sage: latex(I)  # default LaTeX display
\left(0, \pi\right)
sage: I1 = OpenInterval(0, pi, name='I'); I1
Real interval I
sage: latex(I1)
I
sage: I2 = OpenInterval(0, pi, name='I', latex_name=r'\mathcal{I}'); I2
Real interval I
sage: latex(I2)
\mathcal{I}


I is endowed with a canonical chart:

sage: I.canonical_chart()
Chart ((0, pi), (t,))
sage: I.canonical_chart() is I.default_chart()
True
sage: I.atlas()
[Chart ((0, pi), (t,))]


The canonical coordinate is returned by the method canonical_coordinate():

sage: I.canonical_coordinate()
t
sage: t = I.canonical_coordinate()
sage: type(t)
<type 'sage.symbolic.expression.Expression'>


However, it can be obtained in the same step as the interval construction by means of the shortcut I.<names>:

sage: I.<t> = OpenInterval(0, pi)
sage: t
t
sage: type(t)
<type 'sage.symbolic.expression.Expression'>


The trick is performed by the Sage preparser:

sage: preparse("I.<t> = OpenInterval(0, pi)")
"I = OpenInterval(Integer(0), pi, names=('t',)); (t,) = I._first_ngens(1)"


In particular the shortcut can be used to set a canonical coordinate symbol different from 't':

sage: J.<x> = OpenInterval(0, pi)
sage: J.canonical_chart()
Chart ((0, pi), (x,))
sage: J.canonical_coordinate()
x


The LaTeX symbol of the canonical coordinate can be adjusted via the same syntax as a chart declaration (see RealChart):

sage: J.<x> = OpenInterval(0, pi, coordinate=r'x:\xi')
sage: latex(x)
{\xi}
sage: latex(J.canonical_chart())
\left(\left(0, \pi\right),({\xi})\right)


An element of the open interval I:

sage: x = I.an_element(); x
Point on the Real interval (0, pi)
sage: x.coord() # coordinates in the default chart = canonical chart
(1/2*pi,)


As for any manifold subset, a specific element of I can be created by providing a tuple containing its coordinate(s) in a given chart:

sage: x = I((2,)) # (2,) = tuple of coordinates in the canonical chart
sage: x
Point on the Real interval (0, pi)


But for convenience, it can also be created directly from the coordinate:

sage: x = I(2); x
Point on the Real interval (0, pi)
sage: x.coord()
(2,)
sage: I(2) == I((2,))
True


By default, the coordinates passed for the element x are those relative to the canonical chart:

sage: I(2) ==  I((2,), chart=I.canonical_chart())
True


The lower and upper bounds of the interval I:

sage: I.lower_bound()
0
sage: I.upper_bound()
pi


One of the endpoint can be infinite:

sage: J = OpenInterval(1, +oo); J
Real interval (1, +Infinity)
sage: J.an_element().coord()
(2,)


The construction of a subinterval can be performed via the argument ambient_interval of OpenInterval:

sage: J = OpenInterval(0, 1, ambient_interval=I); J
Real interval (0, 1)


However, it is recommended to use the method open_interval() instead:

sage: J = I.open_interval(0, 1); J
Real interval (0, 1)
sage: J.is_subset(I)
True
sage: J.manifold() is I
True


A subinterval of a subinterval:

sage: K = J.open_interval(1/2, 1); K
Real interval (1/2, 1)
sage: K.is_subset(J)
True
sage: K.is_subset(I)
True
sage: K.manifold() is I
True


We have:

sage: I.list_of_subsets()
[Real interval (0, 1), Real interval (0, pi), Real interval (1/2, 1)]
sage: J.list_of_subsets()
[Real interval (0, 1), Real interval (1/2, 1)]
sage: K.list_of_subsets()
[Real interval (1/2, 1)]


As any open subset of a manifold, open subintervals are created in a category of subobjects of smooth manifolds:

sage: J.category()
Join of Category of subobjects of sets and Category of smooth manifolds
over Real Field with 53 bits of precision
sage: K.category()
Join of Category of subobjects of sets and Category of smooth manifolds
over Real Field with 53 bits of precision


On the contrary, I, which has not been created as a subinterval, is in the category of smooth manifolds (see Manifolds):

sage: I.category()
Category of smooth manifolds over Real Field with 53 bits of precision


and we have:

sage: J.category() is I.category().Subobjects()
True


All intervals are parents:

sage: x = J(1/2); x
Point on the Real interval (0, pi)
sage: x.parent() is J
True
sage: y = K(3/4); y
Point on the Real interval (0, pi)
sage: y.parent() is K
True


We have:

sage: x in I, x in J, x in K
(True, True, False)
sage: y in I, y in J, y in K
(True, True, True)


The canonical chart of subintervals is inherited from the canonical chart of the parent interval:

sage: XI = I.canonical_chart(); XI
Chart ((0, pi), (t,))
sage: XI.coord_range()
t: (0, pi)
sage: XJ = J.canonical_chart(); XJ
Chart ((0, 1), (t,))
sage: XJ.coord_range()
t: (0, 1)
sage: XK = K.canonical_chart(); XK
Chart ((1/2, 1), (t,))
sage: XK.coord_range()
t: (1/2, 1)

canonical_chart()

Return the canonical chart defined on self.

OUTPUT:

EXAMPLES:

Canonical chart on the interval $$(0, \pi)$$:

sage: I = OpenInterval(0, pi)
sage: I.canonical_chart()
Chart ((0, pi), (t,))
sage: I.canonical_chart().coord_range()
t: (0, pi)


The symbol used for the coordinate of the canonical chart is that defined during the construction of the interval:

sage: I.<x> = OpenInterval(0, pi)
sage: I.canonical_chart()
Chart ((0, pi), (x,))

canonical_coordinate()

Return the canonical coordinate defined on the interval.

OUTPUT:

• the symbolic variable representing the canonical coordinate

EXAMPLES:

Canonical coordinate on the interval $$(0, \pi)$$:

sage: I = OpenInterval(0, pi)
sage: I.canonical_coordinate()
t
sage: type(I.canonical_coordinate())
<type 'sage.symbolic.expression.Expression'>
sage: I.canonical_coordinate().is_real()
True


The canonical coordinate is the first (unique) coordinate of the canonical chart:

sage: I.canonical_coordinate() is I.canonical_chart()[0]
True


Its default symbol is $$t$$; but it can be customized during the creation of the interval:

sage: I = OpenInterval(0, pi, coordinate='x')
sage: I.canonical_coordinate()
x
sage: I.<x> = OpenInterval(0, pi)
sage: I.canonical_coordinate()
x

inf()

Return the lower bound (infimum) of the interval.

EXAMPLES:

sage: I = OpenInterval(1/4, 3)
sage: I.lower_bound()
1/4
sage: J = OpenInterval(-oo, 2)
sage: J.lower_bound()
-Infinity


An alias of lower_bound() is inf():

sage: I.inf()
1/4
sage: J.inf()
-Infinity

lower_bound()

Return the lower bound (infimum) of the interval.

EXAMPLES:

sage: I = OpenInterval(1/4, 3)
sage: I.lower_bound()
1/4
sage: J = OpenInterval(-oo, 2)
sage: J.lower_bound()
-Infinity


An alias of lower_bound() is inf():

sage: I.inf()
1/4
sage: J.inf()
-Infinity

open_interval(lower, upper, name=None, latex_name=None)

Define an open subinterval of self.

INPUT:

• lower – lower bound of the subinterval (possibly -Infinity)
• upper – upper bound of the subinterval (possibly +Infinity)
• name – (default: None) string; name (symbol) given to the subinterval; if None, the name is constructed from lower and upper
• latex_name – (default: None) string; LaTeX symbol to denote the subinterval; if None, the LaTeX symbol is constructed from lower and upper if name is None, otherwise, it is set to name

OUTPUT:

EXAMPLES:

The interval $$(0, \pi)$$ as a subinterval of $$(-4, 4)$$:

sage: I = OpenInterval(-4, 4)
sage: J = I.open_interval(0, pi); J
Real interval (0, pi)
sage: J.is_subset(I)
True
sage: I.list_of_subsets()
[Real interval (-4, 4), Real interval (0, pi)]


J is considered as an open submanifold of I:

sage: J.manifold() is I
True


The subinterval $$(-4, 4)$$ is I itself:

sage: I.open_interval(-4, 4) is I
True

sup()

Return the upper bound (supremum) of the interval.

EXAMPLES:

sage: I = OpenInterval(1/4, 3)
sage: I.upper_bound()
3
sage: J = OpenInterval(1, +oo)
sage: J.upper_bound()
+Infinity


An alias of upper_bound() is sup():

sage: I.sup()
3
sage: J.sup()
+Infinity

upper_bound()

Return the upper bound (supremum) of the interval.

EXAMPLES:

sage: I = OpenInterval(1/4, 3)
sage: I.upper_bound()
3
sage: J = OpenInterval(1, +oo)
sage: J.upper_bound()
+Infinity


An alias of upper_bound() is sup():

sage: I.sup()
3
sage: J.sup()
+Infinity

class sage.manifolds.differentiable.real_line.RealLine(name='R', latex_name='\Bold{R}', coordinate=None, names=None, start_index=0)

Field of real numbers, as a differentiable manifold of dimension 1 (real line) with a canonical coordinate chart.

INPUT:

• name – (default: 'R') string; name (symbol) given to the real line
• latex_name – (default: r'\Bold{R}') string; LaTeX symbol to denote the real line
• coordinate – (default: None) string defining the symbol of the canonical coordinate set on the real line; if none is provided and names is None, the symbol ‘t’ is used
• names – (default: None) used only when coordinate is None: it must be a single-element tuple containing the canonical coordinate symbol (this is guaranteed if the shortcut <names> is used, see examples below)
• start_index – (default: 0) unique value of the index for vectors and forms on the real line manifold

EXAMPLES:

Constructing the real line without any argument:

sage: R = RealLine() ; R
Real number line R
sage: latex(R)
\Bold{R}


R is a 1-dimensional real smooth manifold:

sage: R.category()
Category of smooth manifolds over Real Field with 53 bits of precision
sage: isinstance(R, sage.manifolds.differentiable.manifold.DifferentiableManifold)
True
sage: dim(R)
1


It is endowed with a canonical chart:

sage: R.canonical_chart()
Chart (R, (t,))
sage: R.canonical_chart() is R.default_chart()
True
sage: R.atlas()
[Chart (R, (t,))]


The instance is unique (as long as the constructor arguments are the same):

sage: R is RealLine()
True
sage: R is RealLine(latex_name='R')
False


The canonical coordinate is returned by the method canonical_coordinate():

sage: R.canonical_coordinate()
t
sage: t = R.canonical_coordinate()
sage: type(t)
<type 'sage.symbolic.expression.Expression'>


However, it can be obtained in the same step as the real line construction by means of the shortcut R.<names>:

sage: R.<t> = RealLine()
sage: t
t
sage: type(t)
<type 'sage.symbolic.expression.Expression'>


The trick is performed by Sage preparser:

sage: preparse("R.<t> = RealLine()")
"R = RealLine(names=('t',)); (t,) = R._first_ngens(1)"


In particular the shortcut is to be used to set a canonical coordinate symbol different from ‘t’:

sage: R.<x> = RealLine()
sage: R.canonical_chart()
Chart (R, (x,))
sage: R.atlas()
[Chart (R, (x,))]
sage: R.canonical_coordinate()
x


The LaTeX symbol of the canonical coordinate can be adjusted via the same syntax as a chart declaration (see RealChart):

sage: R.<x> = RealLine(coordinate=r'x:\xi')
sage: latex(x)
{\xi}
sage: latex(R.canonical_chart())
\left(\Bold{R},({\xi})\right)


The LaTeX symbol of the real line itself can also be customized:

sage: R.<x> = RealLine(latex_name=r'\mathbb{R}')
sage: latex(R)
\mathbb{R}


Elements of the real line can be constructed directly from a number:

sage: p = R(2) ; p
Point on the Real number line R
sage: p.coord()
(2,)
sage: p = R(1.742) ; p
Point on the Real number line R
sage: p.coord()
(1.74200000000000,)


Symbolic variables can also be used:

sage: p = R(pi, name='pi') ; p
Point pi on the Real number line R
sage: p.coord()
(pi,)
sage: a = var('a')
sage: p = R(a) ; p
Point on the Real number line R
sage: p.coord()
(a,)


The real line is considered as the open interval $$(-\infty, +\infty)$$:

sage: isinstance(R, sage.manifolds.differentiable.real_line.OpenInterval)
True
sage: R.lower_bound()
-Infinity
sage: R.upper_bound()
+Infinity


A real interval can be created from R means of the method open_interval():

sage: I = R.open_interval(0, 1); I
Real interval (0, 1)
sage: I.manifold()
Real number line R
sage: R.list_of_subsets()
[Real interval (0, 1), Real number line R]