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Is there a way to find the symbolic matrix associated to an expended quadratic form?

asked 1 year ago

Cyrille gravatar image

Suppose I have a complex quadratic form

Q(px,py)=p2yy2α22pxpyxyαβ+p2xx2β2p2yy2αp2xx2β

Of course all parameters has been declared as variables, that is

var('p_x p_y x y \alpha \beta')

(I use unicode α and β)

Is there a way to find the matrix A such that

Q(p)=pAp ?

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answered 1 year ago

achrzesz gravatar image

The question is answered in https://ask.sagemath.org/question/680...

x,y,α,β=var('x,y,α,β')
P.<U,V> = PolynomialRing(SR, 'U,V')
hes0=(V^2*y^2*α^2 - 2*U*V*x*y*α*β + U^2*x^2*β^2 - V^2*y^2 - U^2*x^2*β)
Q = QuadraticForm.from_polynomial(hes0)
mG=Q.Gram_matrix()
p_x,p_y=var('p_x,p_y')
v=vector([p_x,p_y])
expand(v*(mG*v))

p_y^2*y^2*α^2 - 2*p_x*p_y*x*y*α*β + p_x^2*x^2*β^2 - p_y^2*y^2 - p_x^2*x^2
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Sorry achresz but I receive Q as an expression and I was not able to substitute V to px et U to py. And I do not know why

Cyrille gravatar imageCyrille ( 1 year ago )

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Asked: 1 year ago

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Last updated: May 06 '23