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Non-degeneracy of the pairing (snake relation)

or equivalently

Ref:

We use the Axiom LinearOperator library

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)library CARTEN MONAL PROP LOP
CartesianTensor is now explicitly exposed in frame initial
CartesianTensor will be automatically loaded when needed from
/var/aw/var/LatexWiki/CARTEN.NRLIB/CARTEN
Monoidal is now explicitly exposed in frame initial
Monoidal will be automatically loaded when needed from
/var/aw/var/LatexWiki/MONAL.NRLIB/MONAL
Prop is now explicitly exposed in frame initial
Prop will be automatically loaded when needed from
/var/aw/var/LatexWiki/PROP.NRLIB/PROP
LinearOperator is now explicitly exposed in frame initial
LinearOperator will be automatically loaded when needed from
/var/aw/var/LatexWiki/LOP.NRLIB/LOP

and some convenient notation

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macro Σ(x,i,n)==reduce(+,[x for i in n])
Type: Void
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macro Ξ(f,i,n)==[f for i in n]
Type: Void
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macro sb == subscript
Type: Void
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macro sp == superscript
Type: Void

Let 𝐋 be the domain of 2-dimensional linear operators

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dim:=2
 (1)
Type: PositiveInteger?
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macro ℒ == List
Type: Void
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macro ℚ == Expression Integer
Type: Void
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𝐋 := LinearOperator(OVAR ['1,'2], ℚ)
 (2)
Type: Type
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𝐞:ℒ 𝐋      := basisOut()
 (3)
Type: List(LinearOperator(OrderedVariableList([1,2]),Expression(Integer)))
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𝐝:ℒ 𝐋      := basisIn()
 (4)
Type: List(LinearOperator(OrderedVariableList([1,2]),Expression(Integer)))
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I:𝐋:=[1]   -- identity for composition
 (5)
Type: LinearOperator(OrderedVariableList([1,2]),Expression(Integer))
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X:𝐋:=[2,1] -- twist
 (6)
Type: LinearOperator(OrderedVariableList([1,2]),Expression(Integer))

## Pairing

A scalar product (pairing) is represented by

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U:=Σ(Σ(sp('u,[i,j])*𝐝.i*𝐝.j, i,1..dim), j,1..dim)
 (7)
Type: LinearOperator(OrderedVariableList([1,2]),Expression(Integer))

In general we do not require that it be symmetric.

## Co-pairing

Solve the "twisted snake relation" as a system of linear equations.

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Ω:𝐋:=Σ(Σ(sb('u,[i,j])*𝐞.i*𝐞.j, i,1..dim), j,1..dim)
 (8)
Type: LinearOperator(OrderedVariableList([1,2]),Expression(Integer))
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Í :=
(  I Ω   ) /
(  I X   ) /
(   U I  )
 (9)
Type: LinearOperator(OrderedVariableList([1,2]),Expression(Integer))
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Ì:=
(   Ω I  ) /
(   X I  ) /
(  I U   )
 (10)
Type: LinearOperator(OrderedVariableList([1,2]),Expression(Integer))
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equate(f,g)==map((x,y)+->(x=y),ravel f, ravel g);
Type: Void
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eq1:=equate(Í,I)
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Compiling function equate with type (LinearOperator(
OrderedVariableList([1,2]),Expression(Integer)), LinearOperator(
OrderedVariableList([1,2]),Expression(Integer))) -> List(Equation
(Expression(Integer)))
 (11)
Type: List(Equation(Expression(Integer)))
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eq2:=equate(Ì,I)
 (12)
Type: List(Equation(Expression(Integer)))
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snake:=solve(concat(eq1,eq2),concat Ξ(Ξ(sb('u,[i,j]), i,1..dim), j,1..dim));
Type: List(List(Equation(Expression(Integer))))
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if #snake ~= 1 then error "no solution"
Type: Void
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Ω:=eval(Ω,snake(1))
 (13)
Type: LinearOperator(OrderedVariableList([1,2]),Expression(Integer))
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matrix Ξ(Ξ(Ω/(𝐝.i*𝐝.j), i,1..dim), j,1..dim)
 (14)
Type: Matrix(LinearOperator(OrderedVariableList([1,2]),Expression(Integer)))

This is equivalent to a matrix inverse (transposed!)

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Um:=matrix Ξ(Ξ((𝐞.i*𝐞.j)/U, i,1..dim), j,1..dim)
 (15)
Type: Matrix(LinearOperator(OrderedVariableList([1,2]),Expression(Integer)))
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mU:=inverse map(retract,Um)
 (16)
Type: Union(Matrix(Expression(Integer)),...)
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Ωm:=Σ(Σ(mU(i,j)*(𝐞.i*𝐞.j), i,1..dim), j,1..dim)
 (17)
Type: LinearOperator(OrderedVariableList([1,2]),Expression(Integer))
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-- compare
test(Ω=Ωm)
 (18)
Type: Boolean

Check that the twisted snake relation holds

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test
(  I Ω   )  /
(  I X   )  /
(   U I  )  =  I
 (19)
Type: Boolean
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test
(   Ω I  )  /
(   X I  )  /
(  I U   )  =  I
 (20)
Type: Boolean

## Dimension

Since the "snake" is twisted, dimension is as expected.

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d:=
Ω /
U
 (21)
Type: LinearOperator(OrderedVariableList([1,2]),Expression(Integer))

This "twisted dimension " depends on !

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d':=
Ω /
X /
U
 (22)
Type: LinearOperator(OrderedVariableList([1,2]),Expression(Integer))

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