Elliptic divisibility sequences, universal Weierstrass curve, division polynomial ω #
Definition: WeierstrassCurve_EDSEngine (5fd20a59-e5e9-5133-a421-34f512baf1be), by Claude,
2026-09-05.
Four layers are set up here. (1) Sequence identities. For $W:\mathbb{Z}\to R$ over a commutative
ring, EllSequence.addMulSub W m n $= W((m+n)\,\mathrm{tdiv}\,2)\cdot W((m-n)\,\mathrm{tdiv}\,2)$, and rel₄, rel₆, relFin4, net
are the associated quadratic and quartic expressions, net W p q r s $= W(p+q+s)W(p-q)W(r+s)W(r)-W(p+r+s)W(p-r)W(q+s)W(q)+W(q+r+s)W(q-r)W(p+s)W(p)$; Rel₃ W m n r is the
three‑term elliptic relation at $(m,n,r)$, so that
IsEllipticSequence is equivalent to $\forall m,n,r$, Rel₃.
OddRec/EvenRec are the doubling recursions $W(2m+1)W(1)^3=W(m+2)W(m)^3-W(m-1)W(m+1)^3$ and
$W(2m)W(2)W(1)^2=W(m)(W(m-1)^2W(m+2)-W(m-2)W(m+1)^2)$; invarNum, invarDenom form the quotient
shown to be independent of $n$. The combinatorial predicates HaveSameParity₄, StrictAnti₄ ($0\le d<c<b<a$), avg₄, dMin, cMin, Rel₄OfValid organise the descent from the two recursions to the
full relation. (2) Complementary sequences:
compl₂EDSAux and
compl₂EDS built from preNormEDS (b^4) c d, with
$\mathrm{normEDS}\cdot\mathrm{compl₂EDS}=\mathrm{normEDS}(2m)$; the recursion compl', its odd
extension compl, and complEDS b c d m n with
$\mathrm{normEDS}(m)\cdot\mathrm{complEDS}(m,n)=\mathrm{normEDS}(nm)$; redInvarNum,
redInvarDenom (the latter by cases on $m \bmod 6$) satisfy
$\mathrm{redInvarNum}=\mathrm{redInvarDenom}\cdot(d+b^4)$. Universality is obtained from
Param, universalNormEDS = normEDS (X B) (X C) (X D), and the specialisation $(b,c,d)=(2,3,2)$ at which
normEDS is the identity. (3) The universal curve: Coeff, Univ.curve over
$\mathbb{Z}[A_1,A_2,A_3,A_4,A_6]$, its bivariate polynomial ring Poly, coordinate ring
Univ.Ring, fraction field Univ.Field, the map polyToField, the generic affine and Jacobian
points, the cusp $y^2=x^3$ with its point $(1,1)$, and the evaluation maps specialize, polyEval,
ringEval. (4) Division polynomials: invar $=6X^2+b_2X+b_4$, ψc with
$\psi_n\psi^c_n=\psi_{2n}$, and ωe $n$, characterised by
$2\omega_n+a_1\phi_n\psi_n+a_3\psi_n^3=\psi^c_n$, with $\omega_0=1$, $\omega_1=Y$; then smulX,
smulY, slopeOne give $\phi_n/\psi_n^2$, $\omega_n/\psi_n^3$ over Univ.Field.
Relation to Mathlib. Mathlib supplies normEDS, preNormEDS and the division polynomials
$\psi_n$, $\phi_n$, $\Psi_3$, $\mathrm{pre}\Psi_4$;
compl₂EDS agrees with Mathlib's complEDS₂. The
sequence predicates are Mathlib's IsEllipticSequence, IsDvdSequence and
IsEllipticDvdSequence, with divisibility indexed by integers. The complementary sequence
complEDS, the polynomial ωe, the universal Weierstrass curve with its coordinate ring and
function field, and the specialisation homomorphisms have no Mathlib counterpart here.
Where it is used. The formulas assembled here express multiplication by $n$ on a Weierstrass curve through $\phi_n$, $\omega_n$ and $\psi_n$, first for the generic point of the universal curve and then, by specialisation, over an arbitrary base. They serve the treatment of torsion points of elliptic curves needed for the Galois representations attached to the Frey curve.
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Notation for the affine point group after base change to the indicated coefficient ring.
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The four-variable elliptic-net relation expression.
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The numerator of the elliptic-sequence invariant associated to the two indices.
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Half the sum of the four indices, using integer division.
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The product of sequence values at the half-sum of the first pair and half-difference of the second pair.
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The four-index relation multiplied by a further half-sum and half-difference product.
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- EllSequence.rel₆ W k l a b c d = EllSequence.addMulSub W k l * EllSequence.rel₄ W a b c d
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The least nonnegative index of the same parity as the given index.
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The next nonnegative index of the same parity after dMin.
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- EllSequence.cMin a = EllSequence.dMin a + 2
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The four-index relation vanishes for strictly decreasing nonnegative indices of the same parity.
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- EllSequence.Rel₄OfValid W a b c d = (EllSequence.HaveSameParity₄ a b c d → EllSequence.StrictAnti₄ a b c d → EllSequence.rel₄ W a b c d = 0)
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The four-index relation with its arguments packaged as a function on Fin 4.
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- EllSequence.relFin4 W t = EllSequence.rel₄ W (t 0) (t 1) (t 2) (t 3)
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An auxiliary division-polynomial expression used to define the complementary doubling sequence without division.
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- compl₂EDSAux b c d m = preNormEDS (b ^ 4) c d (m - 2) * preNormEDS (b ^ 4) c d (m + 1) ^ 2 * if Even m then 1 else b
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The complementary doubling sequence whose product with the normalized elliptic sequence gives its doubled-index values.
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The recursively defined complementary sequence on natural indices for a fixed base index.
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- EllSequence.compl' W₁ compl₂ m 0 = 0
- EllSequence.compl' W₁ compl₂ m 1 = 1
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The odd extension to integer indices of the complementary sequence.
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- EllSequence.compl W₁ compl₂ m n = ↑n.sign * EllSequence.compl' W₁ compl₂ m n.natAbs
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The complementary sequence for normalized elliptic divisibility sequences, multiplying the base-index value to the product-index value.
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- EllSequence.complEDS b c d m = EllSequence.compl (normEDS b c d) (compl₂EDS b c d) m
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The normalized elliptic divisibility sequence over the integer polynomial ring in the three parameters.
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The reduced numerator obtained by removing the universal factor from the invariant numerator at first index one.
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- EllSequence.redInvarNum b c d m = compl₂EDS b c d m + normEDS b c d m ^ 3 * b + 2 * compl₂EDSAux b c d m
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The reduced denominator obtained by removing the universal factors from the invariant denominator at first index one.
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The universal Weierstrass curve over the integer polynomial ring in its five coefficients.
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The bivariate polynomial ring for affine coordinates of the universal curve.
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The coordinate ring of the universal Weierstrass curve.
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The fraction field of the universal curve's coordinate ring.
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The map from coordinate polynomials to the function field of the universal curve.
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The universal curve base-changed to its own function field.
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The generic affine point of the universal curve over its function field.
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The generic point of the universal curve in Jacobian coordinates.
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The universal curve base-changed to the coordinate polynomial ring.
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The universal curve base-changed to its coordinate ring.
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The universal curve base-changed to its function field.
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The cuspidal Weierstrass equation y² = x³ over the integers.
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- WeierstrassCurve.cusp = { a₁ := 0, a₂ := 0, a₃ := 0, a₄ := 0, a₆ := 0 }
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Evaluate the five universal coefficient variables at the coefficients of the given curve.
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- W.specialize = (MvPolynomial.aeval fun (t : WeierstrassCurve.Coeff) => WeierstrassCurve.Coeff.rec W.a₁ W.a₂ W.a₃ W.a₄ W.a₆ t).toRingHom
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Evaluate the universal coefficient and coordinate variables at the given curve and coordinates.
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Evaluate the universal coordinate ring at a point satisfying the given curve equation.
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The polynomial 6X² + b₂X + b₄ used in the division-sequence invariant.
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- W.invar = 6 * Polynomial.X ^ 2 + Polynomial.C W.b₂ * Polynomial.X + Polynomial.C W.b₄
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The complementary division polynomial whose product with ψₙ is ψ₂ₙ.
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- W.ψc = compl₂EDS W.ψ₂ (Polynomial.C W.Ψ₃) (Polynomial.C W.preΨ₄)
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The polynomial numerator for the y-coordinate of multiplication by an integer.
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The division sequence of the universal curve viewed in its function field.
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The tangent slope at the generic point of the universal curve.
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The coordinate-polynomial triple [φₙ, ωₙ, ψₙ] for multiplication by an integer.
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The multiplication-coordinate triple mapped to the universal coordinate ring.
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The multiplication-coordinate triple mapped to the universal function field.
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Evaluate the multiplication-coordinate triple at the given affine coordinates.