Algebra of Scalar Fields

The class ScalarFieldAlgebra implements the commutative algebra \(C^0(M)\) of scalar fields on a topological manifold \(M\) over a topological field \(K\). By scalar field, it is meant a continuous function \(M \to K\). The set \(C^0(M)\) 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

  • Travis Scrimshaw (2016): review tweaks

REFERENCES:

class sage.manifolds.scalarfield_algebra.ScalarFieldAlgebra(domain)

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

Commutative algebra of scalar fields on a topological manifold.

If \(M\) is a topological manifold over a topological field \(K\), the commutative algebra of scalar fields on \(M\) is the set \(C^0(M)\) of all continuous maps \(M \to K\). The set \(C^0(M)\) 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 \(C^0(M)\) is constructed is represented by the Symbolic Ring SR, since there is no exact representation of \(\RR\) nor \(\CC\).

INPUT:

  • domain – the topological manifold \(M\) on which the scalar fields are defined

EXAMPLES:

Algebras of scalar fields on the sphere \(S^2\) and on some open subsets of it:

sage: M = Manifold(2, 'M', structure='topological') # 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 scalar fields on the 2-dimensional topological manifold M
sage: W = U.intersection(V)  # S^2 minus the two poles
sage: CW = W.scalar_field_algebra(); CW
Algebra of scalar fields on the Open subset W of the
 2-dimensional topological manifold M

\(C^0(M)\) and \(C^0(W)\) belong to the category of commutative algebras over \(\RR\) (represented here by SymbolicRing):

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 \(C^0(M)\) are scalar fields on \(M\):

sage: CM.an_element()
Scalar field on the 2-dimensional topological 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 \(C^0(W)\) are scalar fields on \(W\):

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

A generic element can be constructed by using a 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 topological 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 scalar fields on the 2-dimensional topological 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 scalar fields on the 2-dimensional topological manifold M
sage: f1 == f
True
sage: M.scalar_field(0, chart='all') == CM.zero()
True

The algebra \(C^0(M)\) coerces to \(C^0(W)\) 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 topological 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 \(C^0(W)\) with elements of \(C^0(M)\), the result being an element of \(C^0(W)\):

sage: s = fW + f
sage: s.parent()
Algebra of scalar fields on the Open subset W of the
 2-dimensional topological 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. 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. 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 topological 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
on W: (u, v) |--> (a*u^2 + a*v^2 + u)/(u^2 + v^2)
sage: h = CM(a+u); h.display()
M --> R
on W: (x, y) |--> (a*x^2 + a*y^2 + x)/(x^2 + y^2)
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
Element

alias of sage.manifolds.scalarfield.ScalarField

one()

Return the unit element of the algebra.

This is nothing but the constant scalar field \(1\) on the manifold, where \(1\) is the unit element of the base field.

EXAMPLES:

sage: M = Manifold(2, 'M', structure='topological')
sage: X.<x,y> = M.chart()
sage: CM = M.scalar_field_algebra()
sage: h = CM.one(); h
Scalar field 1 on the 2-dimensional topological manifold M
sage: h.display()
1: M --> R
   (x, y) |--> 1

The result is cached:

sage: CM.one() is h
True
zero()

Return the zero element of the algebra.

This is nothing but the constant scalar field \(0\) on the manifold, where \(0\) is the zero element of the base field.

EXAMPLES:

sage: M = Manifold(2, 'M', structure='topological')
sage: X.<x,y> = M.chart()
sage: CM = M.scalar_field_algebra()
sage: z = CM.zero(); z
Scalar field zero on the 2-dimensional topological manifold M
sage: z.display()
zero: M --> R
   (x, y) |--> 0

The result is cached:

sage: CM.zero() is z
True