# Algebra of Differentiable Scalar Fields¶

The class DiffScalarFieldAlgebra implements the commutative algebra $$C^k(M)$$ of differentiable scalar fields on a differentiable manifold $$M$$ of class $$C^k$$ over a topological field $$K$$ (in most applications, $$K = \RR$$ or $$K = \CC$$). By differentiable scalar field, it is meant a function $$M\rightarrow K$$ that is $$k$$-times continuously differentiable. $$C^k(M)$$ is an algebra over $$K$$, whose ring product is the pointwise multiplication of $$K$$-valued functions, which is clearly commutative.

AUTHORS:

• Eric Gourgoulhon, Michal Bejger (2014-2015): initial version

REFERENCES:

class sage.manifolds.differentiable.scalarfield_algebra.DiffScalarFieldAlgebra(domain)

Commutative algebra of differentiable scalar fields on a differentiable manifold.

If $$M$$ is a differentiable manifold of class $$C^k$$ over a topological field $$K$$, the commutative algebra of scalar fields on $$M$$ is the set $$C^k(M)$$ of all $$k$$-times continuously differentiable maps $$M\rightarrow K$$. The set $$C^k(M)$$ is an algebra over $$K$$, whose ring product is the pointwise multiplication of $$K$$-valued functions, which is clearly commutative.

If $$K = \RR$$ or $$K = \CC$$, the field $$K$$ over which the algebra $$C^k(M)$$ is constructed is represented by Sage’s Symbolic Ring SR, since there is no exact representation of $$\RR$$ nor $$\CC$$ in Sage.

Via its base class ScalarFieldAlgebra, the class DiffScalarFieldAlgebra inherits from Parent, with the category set to CommutativeAlgebras. The corresponding element class is DiffScalarField.

INPUT:

EXAMPLES:

Algebras of scalar fields on the sphere $$S^2$$ and on some open subset of it:

sage: M = Manifold(2, 'M') # the 2-dimensional sphere S^2
sage: U = M.open_subset('U') # complement of the North pole
sage: c_xy.<x,y> = U.chart() # stereographic coordinates from the North pole
sage: V = M.open_subset('V') # complement of the South pole
sage: c_uv.<u,v> = V.chart() # stereographic coordinates from the South pole
sage: M.declare_union(U,V)   # S^2 is the union of U and V
sage: xy_to_uv = c_xy.transition_map(c_uv, (x/(x^2+y^2), y/(x^2+y^2)),
....:                 intersection_name='W', restrictions1= x^2+y^2!=0,
....:                 restrictions2= u^2+v^2!=0)
sage: uv_to_xy = xy_to_uv.inverse()
sage: CM = M.scalar_field_algebra() ; CM
Algebra of differentiable scalar fields on the 2-dimensional
differentiable manifold M
sage: W = U.intersection(V)  # S^2 minus the two poles
sage: CW = W.scalar_field_algebra() ; CW
Algebra of differentiable scalar fields on the Open subset W of the
2-dimensional differentiable manifold M


$$C^k(M)$$ and $$C^k(W)$$ belong to the category of commutative algebras over $$\RR$$ (represented here by Sage’s Symbolic Ring):

sage: CM.category()
Category of commutative algebras over Symbolic Ring
sage: CM.base_ring()
Symbolic Ring
sage: CW.category()
Category of commutative algebras over Symbolic Ring
sage: CW.base_ring()
Symbolic Ring


The elements of $$C^k(M)$$ are scalar fields on $$M$$:

sage: CM.an_element()
Scalar field on the 2-dimensional differentiable manifold M
sage: CM.an_element().display()  # this sample element is a constant field
M --> R
on U: (x, y) |--> 2
on V: (u, v) |--> 2


Those of $$C^k(W)$$ are scalar fields on $$W$$:

sage: CW.an_element()
Scalar field on the Open subset W of the 2-dimensional differentiable
manifold M
sage: CW.an_element().display()  # this sample element is a constant field
W --> R
(x, y) |--> 2
(u, v) |--> 2


The zero element:

sage: CM.zero()
Scalar field zero on the 2-dimensional differentiable manifold M
sage: CM.zero().display()
zero: M --> R
on U: (x, y) |--> 0
on V: (u, v) |--> 0

sage: CW.zero()
Scalar field zero on the Open subset W of the 2-dimensional
differentiable manifold M
sage: CW.zero().display()
zero: W --> R
(x, y) |--> 0
(u, v) |--> 0


The unit element:

sage: CM.one()
Scalar field 1 on the 2-dimensional differentiable manifold M
sage: CM.one().display()
1: M --> R
on U: (x, y) |--> 1
on V: (u, v) |--> 1

sage: CW.one()
Scalar field 1 on the Open subset W of the 2-dimensional differentiable
manifold M
sage: CW.one().display()
1: W --> R
(x, y) |--> 1
(u, v) |--> 1


A generic element can be constructed as for any parent in Sage, namely by means of the __call__ operator on the parent (here with the dictionary of the coordinate expressions defining the scalar field):

sage: f = CM({c_xy: atan(x^2+y^2), c_uv: pi/2 - atan(u^2+v^2)}); f
Scalar field on the 2-dimensional differentiable manifold M
sage: f.display()
M --> R
on U: (x, y) |--> arctan(x^2 + y^2)
on V: (u, v) |--> 1/2*pi - arctan(u^2 + v^2)
sage: f.parent()
Algebra of differentiable scalar fields on the 2-dimensional
differentiable manifold M


Specific elements can also be constructed in this way:

sage: CM(0) == CM.zero()
True
sage: CM(1) == CM.one()
True


Note that the zero scalar field is cached:

sage: CM(0) is CM.zero()
True


Elements can also be constructed by means of the method scalar_field() acting on the domain (this allows one to set the name of the scalar field at the construction):

sage: f1 = M.scalar_field({c_xy: atan(x^2+y^2), c_uv: pi/2 - atan(u^2+v^2)},
....:                     name='f')
sage: f1.parent()
Algebra of differentiable scalar fields on the 2-dimensional
differentiable manifold M
sage: f1 == f
True
sage: M.scalar_field(0, chart='all') == CM.zero()
True


The algebra $$C^k(M)$$ coerces to $$C^k(W)$$ since $$W$$ is an open subset of $$M$$:

sage: CW.has_coerce_map_from(CM)
True


The reverse is of course false:

sage: CM.has_coerce_map_from(CW)
False


The coercion map is nothing but the restriction to $$W$$ of scalar fields on $$M$$:

sage: fW = CW(f) ; fW
Scalar field on the Open subset W of the 2-dimensional differentiable
manifold M
sage: fW.display()
W --> R
(x, y) |--> arctan(x^2 + y^2)
(u, v) |--> 1/2*pi - arctan(u^2 + v^2)

sage: CW(CM.one()) == CW.one()
True


The coercion map allows for the addition of elements of $$C^k(W)$$ with elements of $$C^k(M)$$, the result being an element of $$C^k(W)$$:

sage: s = fW + f
sage: s.parent()
Algebra of differentiable scalar fields on the Open subset W of the
2-dimensional differentiable manifold M
sage: s.display()
W --> R
(x, y) |--> 2*arctan(x^2 + y^2)
(u, v) |--> pi - 2*arctan(u^2 + v^2)


Another coercion is that from the Symbolic Ring, the parent of all symbolic expressions (cf. SymbolicRing). Since the Symbolic Ring is the base ring for the algebra CM, the coercion of a symbolic expression s is performed by the operation s*CM.one(), which invokes the reflected multiplication operator sage.manifolds.scalarfield.ScalarField._rmul_(). If the symbolic expression does not involve any chart coordinate, the outcome is a constant scalar field:

sage: h = CM(pi*sqrt(2)) ; h
Scalar field on the 2-dimensional differentiable manifold M
sage: h.display()
M --> R
on U: (x, y) |--> sqrt(2)*pi
on V: (u, v) |--> sqrt(2)*pi
sage: a = var('a')
sage: h = CM(a); h.display()
M --> R
on U: (x, y) |--> a
on V: (u, v) |--> a


If the symbolic expression involves some coordinate of one of the manifold’s charts, the outcome is initialized only on the chart domain:

sage: h = CM(a+x); h.display()
M --> R
on U: (x, y) |--> a + x
sage: h = CM(a+u); h.display()
M --> R
on V: (u, v) |--> a + u


If the symbolic expression involves coordinates of different charts, the scalar field is created as a Python object, but is not initialized, in order to avoid any ambiguity:

sage: h = CM(x+u); h.display()
M --> R


TESTS OF THE ALGEBRA LAWS:

Ring laws:

sage: h = CM(pi*sqrt(2))
sage: s = f + h ; s
Scalar field on the 2-dimensional differentiable manifold M
sage: s.display()
M --> R
on U: (x, y) |--> sqrt(2)*pi + arctan(x^2 + y^2)
on V: (u, v) |--> 1/2*pi*(2*sqrt(2) + 1) - arctan(u^2 + v^2)

sage: s = f - h ; s
Scalar field on the 2-dimensional differentiable manifold M
sage: s.display()
M --> R
on U: (x, y) |--> -sqrt(2)*pi + arctan(x^2 + y^2)
on V: (u, v) |--> -1/2*pi*(2*sqrt(2) - 1) - arctan(u^2 + v^2)

sage: s = f*h ; s
Scalar field on the 2-dimensional differentiable manifold M
sage: s.display()
M --> R
on U: (x, y) |--> sqrt(2)*pi*arctan(x^2 + y^2)
on V: (u, v) |--> 1/2*sqrt(2)*(pi^2 - 2*pi*arctan(u^2 + v^2))

sage: s = f/h ; s
Scalar field on the 2-dimensional differentiable manifold M
sage: s.display()
M --> R
on U: (x, y) |--> 1/2*sqrt(2)*arctan(x^2 + y^2)/pi
on V: (u, v) |--> 1/4*sqrt(2)*(pi - 2*arctan(u^2 + v^2))/pi

sage: f*(h+f) == f*h + f*f
True


Ring laws with coercion:

sage: f - fW == CW.zero()
True
sage: f/fW == CW.one()
True
sage: s = f*fW ; s
Scalar field on the Open subset W of the 2-dimensional differentiable
manifold M
sage: s.display()
W --> R
(x, y) |--> arctan(x^2 + y^2)^2
(u, v) |--> 1/4*pi^2 - pi*arctan(u^2 + v^2) + arctan(u^2 + v^2)^2
sage: s/f == fW
True


Multiplication by a number:

sage: s = 2*f ; s
Scalar field on the 2-dimensional differentiable manifold M
sage: s.display()
M --> R
on U: (x, y) |--> 2*arctan(x^2 + y^2)
on V: (u, v) |--> pi - 2*arctan(u^2 + v^2)

sage: 0*f == CM.zero()
True
sage: 1*f == f
True
sage: 2*(f/2) == f
True
sage: (f+2*f)/3 == f
True
sage: 1/3*(f+2*f) == f
True


The Sage test suite for algebras is passed:

sage: TestSuite(CM).run()


It is passed also for $$C^k(W)$$:

sage: TestSuite(CW).run()

Element