Exterior powers of dual free modules

Given a free module \(M\) of finite rank over a commutative ring \(R\) and a positive integer \(p\), the p-th exterior power of the dual of \(M\) is the set \(\Lambda^p(M^*)\) of all alternating forms of degree \(p\) on \(M\), i.e. of all multilinear maps

\[\underbrace{M\times\cdots\times M}_{p\ \; \mbox{times}} \longrightarrow R\]

that vanish whenever any of two of their arguments are equal. Note that \(\Lambda^1(M^*) = M^*\) (the dual of \(M\)).

\(\Lambda^p(M^*)\) is a free module of rank \(\binom{n}{p}\) over \(R\), where \(n\) is the rank of \(M\). Accordingly, exterior powers of free modules are implemented by a class, ExtPowerFreeModule, which inherits from the class FiniteRankFreeModule.

AUTHORS:

  • Eric Gourgoulhon (2015): initial version

REFERENCES:

class sage.tensor.modules.ext_pow_free_module.ExtPowerFreeModule(fmodule, degree, name=None, latex_name=None)

Bases: sage.tensor.modules.finite_rank_free_module.FiniteRankFreeModule

Class for the exterior powers of the dual of a free module of finite rank over a commutative ring.

Given a free module \(M\) of finite rank over a commutative ring \(R\) and a positive integer \(p\), the p-th exterior power of the dual of \(M\) is the set \(\Lambda^p(M^*)\) of all alternating forms of degree \(p\) on \(M\), i.e. of all multilinear maps

\[\underbrace{M\times\cdots\times M}_{p\ \; \mbox{times}} \longrightarrow R\]

that vanish whenever any of two of their arguments are equal. Note that \(\Lambda^1(M^*) = M^*\) (the dual of \(M\)).

\(\Lambda^p(M^*)\) is a free module of rank \(\binom{n}{p}\) over \(R\), where \(n\) is the rank of \(M\). Accordingly, the class ExtPowerFreeModule inherits from the class FiniteRankFreeModule.

This is a Sage parent class, whose element class is FreeModuleAltForm.

INPUT:

  • fmodule – free module \(M\) of finite rank, as an instance of FiniteRankFreeModule
  • degree – positive integer; the degree \(p\) of the alternating forms
  • name – (default: None) string; name given to \(\Lambda^p(M^*)\)
  • latex_name – (default: None) string; LaTeX symbol to denote \(\Lambda^p(M^*)\)

EXAMPLES:

2nd exterior power of the dual of a free \(\ZZ\)-module of rank 3:

sage: M = FiniteRankFreeModule(ZZ, 3, name='M')
sage: e = M.basis('e')
sage: from sage.tensor.modules.ext_pow_free_module import ExtPowerFreeModule
sage: A = ExtPowerFreeModule(M, 2) ; A
2nd exterior power of the dual of the Rank-3 free module M over the
 Integer Ring

Instead of importing ExtPowerFreeModule in the global name space, it is recommended to use the module’s method dual_exterior_power():

sage: A = M.dual_exterior_power(2) ; A
2nd exterior power of the dual of the Rank-3 free module M over the
 Integer Ring
sage: latex(A)
\Lambda^{2}\left(M^*\right)

A is a module (actually a free module) over \(\ZZ\):

sage: A.category()
Category of finite dimensional modules over Integer Ring
sage: A in Modules(ZZ)
True
sage: A.rank()
3
sage: A.base_ring()
Integer Ring
sage: A.base_module()
Rank-3 free module M over the Integer Ring

A is a parent object, whose elements are alternating forms, represented by instances of the class FreeModuleAltForm:

sage: a = A.an_element() ; a
Alternating form of degree 2 on the Rank-3 free module M over the
 Integer Ring
sage: a.display() # expansion with respect to M's default basis (e)
e^0/\e^1
sage: from sage.tensor.modules.free_module_alt_form import FreeModuleAltForm
sage: isinstance(a, FreeModuleAltForm)
True
sage: a in A
True
sage: A.is_parent_of(a)
True

Elements can be constructed from A. In particular, 0 yields the zero element of A:

sage: A(0)
Alternating form zero of degree 2 on the Rank-3 free module M over the
 Integer Ring
sage: A(0) is A.zero()
True

while non-zero elements are constructed by providing their components in a given basis:

sage: e
Basis (e_0,e_1,e_2) on the Rank-3 free module M over the Integer Ring
sage: comp = [[0,3,-1],[-3,0,4],[1,-4,0]]
sage: a = A(comp, basis=e, name='a') ; a
Alternating form a of degree 2 on the Rank-3 free module M over the
 Integer Ring
sage: a.display(e)
a = 3 e^0/\e^1 - e^0/\e^2 + 4 e^1/\e^2

An alternative is to construct the alternating form from an empty list of components and to set the nonzero components afterwards:

sage: a = A([], name='a')
sage: a.set_comp(e)[0,1] = 3
sage: a.set_comp(e)[0,2] = -1
sage: a.set_comp(e)[1,2] = 4
sage: a.display(e)
a = 3 e^0/\e^1 - e^0/\e^2 + 4 e^1/\e^2

The exterior powers are unique:

sage: A is M.dual_exterior_power(2)
True

The exterior power \(\Lambda^1(M^*)\) is nothing but \(M^*\):

sage: M.dual_exterior_power(1) is M.dual()
True
sage: M.dual()
Dual of the Rank-3 free module M over the Integer Ring
sage: latex(M.dual())
M^*

Since any tensor of type (0,1) is a linear form, there is a coercion map from the set \(T^{(0,1)}(M)\) of such tensors to \(M^*\):

sage: T01 = M.tensor_module(0,1) ; T01
Free module of type-(0,1) tensors on the Rank-3 free module M over the
 Integer Ring
sage: M.dual().has_coerce_map_from(T01)
True

There is also a coercion map in the reverse direction:

sage: T01.has_coerce_map_from(M.dual())
True

For a degree \(p\geq 2\), the coercion holds only in the direction \(\Lambda^p(M^*)\rightarrow T^{(0,p)}(M)\):

sage: T02 = M.tensor_module(0,2) ; T02
Free module of type-(0,2) tensors on the Rank-3 free module M over the
 Integer Ring
sage: T02.has_coerce_map_from(A)
True
sage: A.has_coerce_map_from(T02)
False

The coercion map \(T^{(0,1)}(M) \rightarrow M^*\) in action:

sage: b = T01([-2,1,4], basis=e, name='b') ; b
Type-(0,1) tensor b on the Rank-3 free module M over the Integer Ring
sage: b.display(e)
b = -2 e^0 + e^1 + 4 e^2
sage: lb = M.dual()(b) ; lb
Linear form b on the Rank-3 free module M over the Integer Ring
sage: lb.display(e)
b = -2 e^0 + e^1 + 4 e^2

The coercion map \(M^* \rightarrow T^{(0,1)}(M)\) in action:

sage: tlb = T01(lb) ; tlb
Type-(0,1) tensor b on the Rank-3 free module M over the Integer Ring
sage: tlb == b
True

The coercion map \(\Lambda^2(M^*)\rightarrow T^{(0,2)}(M)\) in action:

sage: ta = T02(a) ; ta
Type-(0,2) tensor a on the Rank-3 free module M over the Integer Ring
sage: ta.display(e)
a = 3 e^0*e^1 - e^0*e^2 - 3 e^1*e^0 + 4 e^1*e^2 + e^2*e^0 - 4 e^2*e^1
sage: a.display(e)
a = 3 e^0/\e^1 - e^0/\e^2 + 4 e^1/\e^2
sage: ta.symmetries() # the antisymmetry is of course preserved
no symmetry;  antisymmetry: (0, 1)
Element

alias of FreeModuleAltForm

base_module()

Return the free module on which self is constructed.

OUTPUT:

  • instance of FiniteRankFreeModule representing the free module on which the exterior power is defined.

EXAMPLES:

sage: M = FiniteRankFreeModule(ZZ, 5, name='M')
sage: A = M.dual_exterior_power(2)
sage: A.base_module()
Rank-5 free module M over the Integer Ring
sage: A.base_module() is M
True
degree()

Return the degree of self.

OUTPUT:

  • integer \(p\) such that self is the exterior power \(\Lambda^p(M^*)\)

EXAMPLES:

sage: M = FiniteRankFreeModule(ZZ, 5, name='M')
sage: A = M.dual_exterior_power(2)
sage: A.degree()
2
sage: M.dual_exterior_power(4).degree()
4