Reconstruction of Algebraic Forms

This module reconstructs algebraic forms from the values of their invariants. Given a set of (classical) invariants, it returns a form that attains this values as invariants (up to scaling).

AUTHORS:

• Jesper Noordsij (2018-06): initial version

sage.rings.invariants.reconstruction.binary_cubic_coefficients_from_invariants(discriminant, invariant_choice='default')

Reconstruct a binary cubic from the value of its discriminant.

INPUT:

• discriminant – The value of the discriminant of the binary cubic.

• invariant_choice – The type of invariants provided. The accepted options are 'discriminant' and 'default', which are the same. No other options are implemented.

OUTPUT:

A set of coefficients of a binary cubic, whose discriminant is equal to the given discriminant up to a scaling.

EXAMPLES:

sage: from sage.rings.invariants.reconstruction import binary_cubic_coefficients_from_invariants
sage: coeffs = binary_cubic_coefficients_from_invariants(1)
sage: coeffs
(0, 1, -1, 0)
sage: R.<x> = QQ[]
sage: R(coeffs).discriminant()
1

The two non-equivalent cubics \(x^3\) and \(x^2*z\) with discriminant 0 can’t be distinguished based on their discriminant, hence an error is raised:

sage: binary_cubic_coefficients_from_invariants(0)
Traceback (most recent call last):
...
ValueError: no unique reconstruction possible for binary cubics with a double root

sage.rings.invariants.reconstruction.binary_quadratic_coefficients_from_invariants(discriminant, invariant_choice='default')

Reconstruct a binary quadratic from the value of its discriminant.

INPUT:

• discriminant – The value of the discriminant of the binary quadratic.

• invariant_choice – The type of invariants provided. The accepted options are 'discriminant' and 'default', which are the same. No other options are implemented.

OUTPUT:

A set of coefficients of a binary quadratic, whose discriminant is equal to the given discriminant up to a scaling.

EXAMPLES:

sage: from sage.rings.invariants.reconstruction import binary_quadratic_coefficients_from_invariants
sage: quadratic = invariant_theory.binary_form_from_invariants(2, [24]) # indirect doctest
sage: quadratic
Binary quadratic with coefficients (1, -6, 0)
sage: quadratic.discriminant()
24
sage: binary_quadratic_coefficients_from_invariants(0)
(1, 0, 0)

sage.rings.invariants.reconstruction.binary_quintic_coefficients_from_invariants(invariants, K=None, invariant_choice='default', scaling='none')

Reconstruct a binary quintic from the values of its (Clebsch) invariants.

INPUT:

• invariants – A list or tuple of values of the three or four invariants. The default option requires the Clebsch invariants \(A\), \(B\), \(C\) and \(R\) of the binary quintic.

• K – The field over which the quintic is defined.

• invariant_choice – The type of invariants provided. The accepted options are 'clebsch' and 'default', which are the same. No other options are implemented.

• scaling – How the coefficients should be scaled. The accepted values are 'none' for no scaling, 'normalized' to scale in such a way that the resulting coefficients are independent of the scaling of the input invariants and 'coprime' which scales the input invariants by dividing them by their gcd.

OUTPUT:

A set of coefficients of a binary quintic, whose invariants are equal to the given invariants up to a scaling.

EXAMPLES:

First we check the general case, where the invariant \(M\) is non-zero:

sage: R.<x0, x1> = QQ[]
sage: p = 3*x1^5 + 6*x1^4*x0 + 3*x1^3*x0^2 + 4*x1^2*x0^3 - 5*x1*x0^4 + 4*x0^5
sage: quintic = invariant_theory.binary_quintic(p, x0, x1)
sage: invs = quintic.clebsch_invariants(as_tuple=True)
sage: reconstructed = invariant_theory.binary_form_from_invariants(5, invs, variables=quintic.variables()) # indirect doctest
sage: reconstructed
Binary quintic with coefficients (9592267437341790539005557/244140625000000,
2149296928207625556323004064707/610351562500000000,
11149651890347700974453304786783/76293945312500000,
122650775751894638395648891202734239/47683715820312500000,
323996630945706528474286334593218447/11920928955078125000,
1504506503644608395841632538558481466127/14901161193847656250000)

We can see that the invariants of the reconstructed form match the ones of the original form by scaling the invariants \(B\) and \(C\):

sage: scale = invs[0]/reconstructed.A_invariant()
sage: invs[1] == reconstructed.B_invariant()*scale^2
True
sage: invs[2] == reconstructed.C_invariant()*scale^3
True

If we compare the form obtained by this reconstruction to the one found by letting the covariants \(\alpha\) and \(\beta\) be the coordinates of the form, we find the forms are the same up to a power of the determinant of \(\alpha\) and \(\beta\):

sage: alpha = quintic.alpha_covariant()
sage: beta = quintic.beta_covariant()
sage: g = matrix([[alpha(x0=1,x1=0),alpha(x0=0,x1=1)],[beta(x0=1,x1=0),beta(x0=0,x1=1)]])^-1
sage: transformed = tuple([g.determinant()^-5*x for x in quintic.transformed(g).coeffs()])
sage: transformed == reconstructed.coeffs()
True

This can also be seen by computing the \(\alpha\) covariant of the obtained form:

sage: reconstructed.alpha_covariant().coefficient(x1)
0
sage: reconstructed.alpha_covariant().coefficient(x0) != 0
True

If the invariant \(M\) vanishes, then the coefficients are computed in a different way:

sage: [A,B,C] = [3,1,2]
sage: M = 2*A*B - 3*C
sage: M
0
sage: from sage.rings.invariants.reconstruction import binary_quintic_coefficients_from_invariants
sage: reconstructed = binary_quintic_coefficients_from_invariants([A,B,C])
sage: reconstructed
(-66741943359375/2097152,
 -125141143798828125/134217728,
 0,
 52793920040130615234375/34359738368,
 19797720015048980712890625/1099511627776,
 -4454487003386020660400390625/17592186044416)
sage: newform = sum([ reconstructed[i]*x0^i*x1^(5-i) for i in range(6) ])
sage: newquintic = invariant_theory.binary_quintic(newform, x0, x1)
sage: scale = 3/newquintic.A_invariant()
sage: [3, newquintic.B_invariant()*scale^2, newquintic.C_invariant()*scale^3]
[3, 1, 2]

Several special cases:

sage: quintic = invariant_theory.binary_quintic(x0^5 - x1^5, x0, x1)
sage: invs = quintic.clebsch_invariants(as_tuple=True)
sage: binary_quintic_coefficients_from_invariants(invs)
(1, 0, 0, 0, 0, 1)
sage: quintic = invariant_theory.binary_quintic(x0*x1*(x0^3-x1^3), x0, x1)
sage: invs = quintic.clebsch_invariants(as_tuple=True)
sage: binary_quintic_coefficients_from_invariants(invs)
(0, 1, 0, 0, 1, 0)
sage: quintic = invariant_theory.binary_quintic(x0^5 + 10*x0^3*x1^2 - 15*x0*x1^4, x0, x1)
sage: invs = quintic.clebsch_invariants(as_tuple=True)
sage: binary_quintic_coefficients_from_invariants(invs)
(1, 0, 10, 0, -15, 0)
sage: quintic = invariant_theory.binary_quintic(x0^2*(x0^3 + x1^3), x0, x1)
sage: invs = quintic.clebsch_invariants(as_tuple=True)
sage: binary_quintic_coefficients_from_invariants(invs)
(1, 0, 0, 1, 0, 0)
sage: quintic = invariant_theory.binary_quintic(x0*(x0^4 + x1^4), x0, x1)
sage: invs = quintic.clebsch_invariants(as_tuple=True)
sage: binary_quintic_coefficients_from_invariants(invs)
(1, 0, 0, 0, 1, 0)

For fields of characteristic 2, 3 or 5, there is no reconstruction implemented. This is part of trac ticket #26786.:

sage: binary_quintic_coefficients_from_invariants([3,1,2], K=GF(5))
Traceback (most recent call last):
...
NotImplementedError: no reconstruction of binary quintics implemented for fields of characteristic 2, 3 or 5