The genus of an exact lattice has a distinct type SymGen which holds a representative lattice, and the local data defining the genus. Each genus consists of 2n spinor genera, for some integer n, typically 1. The spinor genera share the same type SymGen. Unlike the genus, the spinor genus is not determined solely by the local data of the genus, so the cached representative is necessary to define the spinor class.
Equality testing of genera is fast, since this requires only a comparison of the canonical local information. It is also possible to enumerate representatives of all equivalences classes in a genus or spinor genus. This is done by a process of exploration of the p-neighbour graph, for an appropriate prime p. The neighbouring functions can be applied to individual lattices to find p-neighbours or the closure under the p-neighbour process. Functions for computing and comparing the local p-adic equivalence classes of lattices, mediated by the type SymGenLoc.
Given an exact lattice L or a spinor genus G this function returns the genus of L. If given a genus the function returns G itself.
Given an exact lattice L, returns the spinor genus of L.
Given a genus G, returns the sequence of spinor genera. If G is a spinor genus, then this function returns the sequence consisting of G itself.
Returns a representative lattice for the genus symbol G.
Returns true if and only if G is a spinor genus. This is the negation of IsGenus(G).
Returns true if and only if G is a genus. This is the negation of IsSpinorGenus(G).
Returns the determinant of the genus symbol G.
Returns the sequence of p-adic genera of the genus symbol G.
Returns a representative lattice for the genus symbol G.
Given two genus symbols, return true if and only if they represent the same genus. This computation is fast for genera, but currently for spinor genera invokes a call to Representatives.
The number of isometry classes in the genus or spinor genus G.Enumeration of isometry classes is done by an explicit call to Representatives, so that #G is an expensive computation.
Return the spinor characters of the genus symbol G as a sequence of Dirichlet characters whose kernels intersect exactly in the group of automorphous numbers. Consult Conway and Sloane [JC98] for precise definitions and significance of the spinor kernel and automorphous numbers.
Return the spinor generators of the genus symbol G as a sequence of primes which generate the group of spinor norms. The primes generate a group dual to that generated by the spinor characters.
A set of integer representatives of the p-adic square classes in the image of the spinor norm of the lattice L (respectively the genus symbol G).
Returns true if and only if p is coprime to 2 and the determinant, and p is the norm of an element of the spinor kernel of G.
Return the prime p for which G represents the p-adic genus.
Returns a canonical representative lattice of the p-adic genus G, with Gram matrix in Jordan form. For odd p the Jordan form is diagonalized.
This function returns a canonical p-adic representative of the determinant of the p-adic genus G. The determinant is well-defined only up to squares.
Return the dimension of the p-adic genus G.
Given local genus symbols G1 and G2, return true if and only if they have the same prime and the same canonical Jordan form.
Let L be an integral lattice, p a prime which does not divide (Determinant)(L) and v a vector in L - pL with (v, v) ∈p2 Z. The p-neighbour of L with respect to v is the lattice generated by Lv and p - 1 v, where Lv := { x ∈L | (x, v) ∈p Z }.See [Kne57] for the original definition and [SP91] for a generalization of the neighbouring method.
For an integral lattice L and prime p, returns the sequence of p-neighbours of L.
Bound: RngIntElt Default: 2^{32}
For an integral lattice L and prime p, returns the sequence of lattices obtained by transitive closure of the p-neighbours of L.Note that neighbours with respect to two vectors v1, v2 whose images in L/pL lie in the same projective orbit of (Aut)(L) on L/pL are isometric. Therefore only projective orbit representatives of the action of (Aut)(L) on L/pL are used. The large number of orbits restricts the complexity of this algorithm, hence the function gives an error if p^((Rank)(L)) is greater than Bound, by default set to 232.
Bound: RngIntElt Default: 2^{32}
Depth: RngIntElt Default:
For an exact lattice L with genus or spinor genus G, this function enumerates the isometry classes in G by constructing the p-neighbour closure (up to isometry). This construction used using an appropriate prime or primes p not dividing the determinant of L. For the genus, sufficiently many primes p are chosen to generate the full image, modulo the spinor kernel, of each character defining the spinor kernel. The parameters are exactly as for the NeighbourClosure function.
For a genus or spinor genus G, this function determines the adjacency matrix of the p-neighbour graph on the representative classes for G. The integer p must be prime, and if G is a spinor genus, then an error ensues if p is not an automorphous number for G.
> Z8 := StandardLattice(8); > v := Z8 ! [1,1,1,1,1,1,1,1]; > E8 := Neighbour(Z8, v, 2); > E8; Lattice of rank 8 and degree 8 Basis: ( 2 0 0 0 0 0 0 2) ( 2 0 0 0 0 0 0 -2) ( 1 1 -1 1 1 -1 1 1) ( 1 1 -1 -1 -1 -1 -1 -1) ( 1 -1 -1 -1 -1 -1 1 -1) ( 0 0 2 0 0 0 0 2) ( 0 0 0 0 2 0 0 2) ( 0 0 0 0 0 2 0 2) Basis denominator: 2The so-obtained lattice is in fact identical to the one returned by the standard construction.
> L := Lattice("E", 8);
> L;
Lattice of rank 8 and degree 8
Basis:
( 4 0 0 0 0 0 0 0)
(-2 2 0 0 0 0 0 0)
( 0 -2 2 0 0 0 0 0)
( 0 0 -2 2 0 0 0 0)
( 0 0 0 -2 2 0 0 0)
( 0 0 0 0 -2 2 0 0)
( 0 0 0 0 0 -2 2 0)
( 1 1 1 1 1 1 1 1)
Basis Denominator: 2
> E8 eq L;
true
We use a combination of the automorphism group, isometry and neighbouring functions. The idea is that the neighbouring graph spans the full genus which therefore can be computed by successively generating neighbours and checking them for isometry with already known ones. The automorphism group comes into play, since neighbours with respect to vectors in the same projective orbit under the automorphism group are isometric.
> L := CoordinateLattice(Lattice("Kappa", 12));
> G := AutomorphismGroup(L);
> G2 := ChangeRing(G, GF(2));
> O := LineOrbits(G2);
> [ Norm(L!Rep(o).1) : o in O ];
[ 4, 8, 10 ]
Hence only the first and second orbits give rise to a 2-neighbour.
To obtain an even neighbour, the second vector has to be adjusted
by an element of 2 * L such that it has norm divisible by 8.
> v1 := L ! Rep(O[1]).1;
> v1 +:= 2 * Rep({ u : u in Basis(L) | (v1,u) mod 2 eq 1 });
> v2 := L ! Rep(O[2]).1;
> Norm(v1), Norm(v2);
16 8
> L1 := Neighbour(L, v1, 2);
> L2 := Neighbour(L, v2, 2);
> bool := IsIsometric(L, L1); bool;
true
> bool := IsIsometric(L, L2); bool;
false
So we obtain only one non-isometric even neighbour of L.
To obtain the full genus we can now proceed with L2 in the
same way, and do this with the following function EvenGenus.
Note that this function is simply one component of the function
GenusRepresentatives.
> function EvenGenus(L)
> // Start with the lattice L
> Lambda := [ CoordinateLattice(LLL(L)) ];
> cand := 1;
> while cand le #Lambda do
> L := Lambda[cand];
> G := ChangeRing( AutomorphismGroup(L), GF(2) );
> // Get the projective orbits on L/2L
> O := LineOrbits(G);
> for o in O do
> v := L ! Rep(o).1;
> if Norm(v) mod 4 eq 0 then
> // Adjust the vector such that its norm is divisible by 8
> if not Norm(v) mod 8 eq 0 then
> v +:= 2 * Rep({ u : u in Basis(L) | (v,u) mod 2 eq 1 });
> end if;
> N := LLL(Neighbour(L, v, 2));
> new := true;
> for i in [1..#Lambda] do
> if IsIsometric(Lambda[i], N) then
> new := false;
> break i;
> end if;
> end for;
> if new then
> Append(~Lambda, CoordinateLattice(N));
> end if;
> end if;
> end for;
> cand +:= 1;
> end while;
> return Lambda;
> end function;
>
> time Lambda := EvenGenus(L);
Time: 9.300
> #Lambda;
10
> [ Minimum(L) : L in Lambda ];
[ 4, 2, 2, 2, 2, 2, 2, 2, 2, 2 ]
> &+[ 1/#AutomorphismGroup(L) : L in Lambda ];
4649359/4213820620800
We see that the genus consists of 10 classes of lattices where only the
Coxeter-Todd lattice has minimum 4 and get the mass of the genus as
4649359/4213820620800.