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# 25 Integral matrices and lattices

### Sections

1. Normal Forms over the Integers
2. Decompositions
3. Lattice Reduction
4. Orthogonal Embeddings

## 25.1 Normal Forms over the Integers

- `DiagonalizeIntMatNormDriven(` mat`) F`

  `DiagonalizeIntMatNormDriven` diagonalizes the integer matrix mat.

  It tries to keep the entries small through careful selection of pivots.

  First it selects a nonzero entry for which the product of row and column norm is minimal (this need not be the entry with minimal absolute value). Then it brings this pivot to the upper left corner and makes it positive.

  Next it subtracts multiples of the first row from the other rows, so that the new entries in the first column have absolute value at most pivot/2. Likewise it subtracts multiples of the 1st column from the other columns.

  If afterwards not all new entries in the first column and row are zero, then it selects a new pivot from those entries (again driven by product of norms) and reduces the first column and row again.

  If finally all offdiagonal entries in the first column and row are zero, then it starts all over again with the submatrix ``mat`[2..][2..]`.

  The original idea is explained in HM97.

      gap> m:=[[14,20],[6,9]];;
      gap> DiagonalizeIntMatNormDriven(m);
      gap> m;
      [ [ 2, 0 ], [ 0, 3 ] ]

- `SNFNormDriven(` mat`[,` trans`] ) O`
- `SNFChouCollins(` mat`[,` trans`] ) O`
- `SNFLLLDriven(` mat`[,` trans`] ) O`

  These operations have been superceded for most purposes by `NormalFormIntMat` (see NormalFormIntMat) which should in most cases be faster than any of them, and produce smaller transforming matrix entries.

  These operations compute the Smith normal form of a matrix with integer entries, using the strategy specified in the name. If no optional argument trans is given mat must be a mutable matrix which will be changed by the algorithm.

  If the optional integer argument trans is given, it determines which transformation matrices will be computed. It is interpreted binary as:

  1
    Row transformations.

  2
    Inverse row transformations.

  4
    Column transformations.

  8
    Inverse column transformations.

  The operation then returns a record with the component `normal` containing the computed normal form and optional components `rowtrans`, `rowinverse`, `coltrans`, and `invcoltrans` which hold the computed transformation matrices. Note, if trans is given the operation does not change mat.

  This functionality is still to be fully implemented for SNF with transforms. However, `NormalFormIntMat` performs this calculation.

- `SNFofREF(` mat`) O`

  Computes the Smith Normal Form of an integer matrix in row echelon (RE) form. Caveat -- No testing is done to ensure that mat is in RE form.

- `HNFNormDriven(` mat`[,` trans`[,` reduction`]] ) O`
- `HNFChouCollins(` mat`[,` trans`[,` reduction`]] ) O`
- `HNFLLLDriven(` mat`[,` trans`[,` reduction`]] ) O`

  These operations have been superceded for most purposes by `NormalFormIntMat` (see NormalFormIntMat) which should in most cases be faster than any of them, and produce smaller transforming matrix entries.

  These operations compute the Hermite normal form of a matrix with integer entries, using the strategy specified in the name. If no optional argument trans is given mat must be a mutable matrix which will be changed by the algorithm.

  If the optional integer argument trans is given, it determines which transformation matrices will be computed. It is interpreted binary as for the Smith normal form (see SNFNormDriven) but note that only row operations are performed. The function then returns a record with components as specified for the Smith normal form.

  If the further optional argument reduction (a rational in the range `[0..1]`) is given, it specifies which representatives are used for entries modulo c when cleaning column entries to the top. Off-diagonal entries are reduced to the range ë c(r-1)û¼ë cr û, where r is the value of reduction. If reduction is not given, a value of 1 is assumed. Note, if trans is given the operation does not change mat.

      gap> m:=[ [ 14, 20 ], [ 6, 9 ] ];;
      gap> HNFNormDriven(m);
      [ [ 2, 2 ], [ 0, 3 ] ]
      gap> m;
      [ [ 2, 2 ], [ 0, 3 ] ]

      gap> m:=[[14,20],[6,9]];;
      gap> HNFNormDriven(m,1);
      rec( normal := [ [ 2, 2 ], [ 0, 3 ] ], rowtrans := [ [ 1, -2 ], [ -3, 7 ] ] )
      gap> m;
      [ [ 14, 20 ], [ 6, 9 ] ]
      gap> last2.rowtrans*m;
      [ [ 2, 2 ], [ 0, 3 ] ]

- `TriangulizeIntegerMat(` mat`[,` trans`] ) O`

  Computes an upper triangular form of a matrix with integer entries. If no optional argument trans is given mat must be a mutable matrix which will be changed by the algorithm.

  If the optional integer argument trans is given, it determines which transformation matrices will be computed. It is interpreted binary as for the Smith normal form (see SNFNormDriven) but note that only row operations are performed. The function then returns a record with components as specified for the Smith normal form. Note, if trans is given the operation does not change mat.

- `SmithNormalFormIntegerMat(` mat`) O`
- `SmithNormalFormIntegerMatTransforms(` mat`) O`
- `SmithNormalFormIntegerMatInverseTransforms(` mat`) O`

  The Smith Normal Form,S, of an integer matrix A is the unique equivalent diagonal form with Si dividing Sj for i < j. There exist unimodular integer matrices P, Q such that PAQ = S·

  These operations compute the Smith normal form of a matrix mat with integer entries. The operations will try to select a suitable strategy. The first operation returns a new immutable matrix in the Smith normal form. The other operations also compute matrices for the row and column transformations or inverses thereof respectively. They return a record with the component `normal` containing the computed normal form and optional components `rowtrans` and `coltrans`, or `invrowtrans` and `invcoltrans` which hold the computed transformation matrices.

      gap> m:=[[14,20],[6,9]];
      [ [ 14, 20 ], [ 6, 9 ] ]
      gap> SmithNormalFormIntegerMat(m);
      [ [ 1, 0 ], [ 0, 6 ] ]

- `HermiteNormalFormIntegerMat(` mat`[,` reduction`] ) O`
- `HermiteNormalFormIntegerMatTransforms(` mat`[,` reduction`] ) O`
- `HermiteNormalFormIntegerMatInverseTransforms(` mat`[,` reduction`] ) O`

  The Hermite Normal Form, H of an integer matrix, A is a row equivalent upper triangular form such that all off-diagonal entries are reduced modulo the diagonal entry of the column they are in. There exists a unique unimodular matrix Q such that QA = H.

  These operations compute the Hermite normal form of a matrix mat with integer entries. The operations will try to select a suitable strategy. The first operation returns a immutable matrix which is the Hermite normal form of mat.

  The other two operations also compute matrices for the row transformations or inverses respectively. They return a record with the component `normal` containing the computed normal form and optional components `rowtrans` or 'invrowtrans' which hold the computed transformation matrix.

  If the optional argument reduction (a rational in the range `[0..1]`) is given, it specifies which representatives are used for entries modulo c when cleaning column entries to the top. Off-diagonal entries are reduced to the range ë c(r-1)û¼ë cr û where r is the value of reduction. If reduction is not given, a value of 1 is assumed.

      gap> m;
      [ [ 14, 20 ], [ 6, 9 ] ]
      gap> HermiteNormalFormIntegerMat(m);
      [ [ 2, 2 ], [ 0, 3 ] ]
      gap> HermiteNormalFormIntegerMatTransforms(m);
      rec( normal := [ [ 2, 2 ], [ 0, 3 ] ], rowtrans := [ [ 1, -2 ], [ -3, 7 ] ] )

- `TriangulizedIntegerMat(` mat`[,` trans`] ) O`
- `TriangulizedIntegerMatTransform(` mat`[,` trans`] ) O`
- `TriangulizedIntegerMatInverseTransform(` mat`[,` trans`] ) O`

  The first operation computes a row equivalent upper triangular form of a matrix mat with integer entries. It returns an immutable matrix in upper triangular form.

  The other two operations also compute matrices for the row transformations or inverses respectively. They return a record with the component `normal` containing the computed normal form and optional components `rowtrans` or `invrowtrans` which hold the computed transformation matrix.

- `NormalFormIntMat(` mat`,` options`) O`

  This general operation for computation of various Normal Forms is probably the most efficient.

  Options bit values:

  0/1
    Triangular Form / Smith Normal Form.

  2
    Reduce off diagonal entries.

  4
    Row Transformations.

  8
    Col Transformations.

  Compute a Triangular, Hermite or Smith form of the n ×m integer input matrix A. Optionally, compute n ×n and m ×m unimodular transforming matrices Q, P which satisfy QA = H or QAP = S. The routines used are based on work by Arne Storjohann and were implemented in GAP 4 by A. Storjohann and R. Wainwright.

  Note option is a value ranging from 0 - 15 but not all options make sense (eg reducing off diagonal entries with SNF option selected already). If an option makes no sense it is ignored.

  Returns a record with component `normal` containing the computed normal form and optional components `rowtrans` and/or `coltrans` which hold the respective transformation matrix. Also in the record are components holding the sign of the determinant, signdet, and the Rank of the matrix, rank.

      gap> m:=[[14,20],[6,9]];;
      gap> NormalFormIntMat(m,0);  # Triangular, no transforms
      rec( normal := [ [ 2, 2 ], [ 0, 3 ] ], rank := 2, signdet := 1 )

      gap> NormalFormIntMat(m,6);  # Hermite Normal Form with row transforms
      rec( normal := [ [ 2, 2 ], [ 0, 3 ] ], rank := 2, signdet := 1,
        rowtrans := [ [ 1, -2 ], [ -3, 7 ] ] )

      gap> NormalFormIntMat(m,13); # Smith Normal Form with both transforming matrices
      rec( normal := [ [ 1, 0 ], [ 0, 6 ] ], rank := 2, signdet := 1,
        rowtrans := [ [ -11, 25 ], [ -15, 34 ] ],
        coltrans := [ [ 1, -5 ], [ 1, -4 ] ] )
      gap> last.rowtrans*m*last.coltrans;
      [ [ 1, 0 ], [ 0, 6 ] ]

  ## 25.2 Decompositions

  For computing the decomposition of a vector of integers into the rows of a matrix of integers, with integral coefficients, one can use p-adic approximations, as follows.

  Let A be a square integral matrix, and p an odd prime. The reduction of A modulo p is [`(A)], its entries are chosen in the interval [-[(p-1)/2], [(p-1)/2]]. If [`(A)] is regular over the field with p elements, we can form A¢ = [`(A)]-1. Now we consider the integral linear equation system x A = b, i.e., we look for an integral solution x. Define b0 = b, and then iteratively compute

  xi = (bi A¢) mod p, bi+1 =

  1

  p

  (bi - xi A), i = 0, 1, 2, ¼.

  By induction, we get

  pi+1 bi+1 + | æ è |  | i å j=0 | pj xj | ö ø | A = b.

  If there is an integral solution x then it is unique, and there is an index l such that bl+1 is zero and x = åj=0l pj xj.

  There are two useful generalizations of this idea. First, A need not be square; it is only necessary that there is a square regular matrix formed by a subset of columns of A. Second, A does not need to be integral; the entries may be cyclotomic integers as well, in this case one can replace each column of A by the columns formed by the coefficients w.r.t. an integral basis (which are integers). Note that this preprocessing must be performed compatibly for A and b.

  GAP provides the following functions for this purpose (see also InverseMatMod).

- `Decomposition(` A`,` B`,` depth`) F`
- `Decomposition(` A`,` B`, "nonnegative" ) F`

  For a m ×n matrix A of cyclotomics that has rank m £ n, and a list B of cyclotomic vectors, each of length n, `Decomposition` tries to find integral solutions of the linear equation systems ``x`*` A`=` B`[i]`, by computing the p-adic series of hypothetical solutions.

  `Decomposition(` A`,` B`,` depth`)`, where depth is a nonnegative integer, computes for each vector ``B`[i]` the initial part åk=0depth xk pk, with all xk vectors of integers with entries bounded by ±[(p-1)/2]. The prime p is 83 first; if the reduction of A modulo p is singular, the next prime is chosen automatically.

  A list X is returned. If the computed initial part for ``x`*` A`=` B`[i]` is a solution, we have ``X`[i] =` x``, otherwise ``X`[i] = fail`.

  `Decomposition(` A`,` B`, "nonnegative" )` assumes that the solutions have only nonnegative entries, and that the first column of A consists of positive integers. This is satisfied, e.g., for the decomposition of ordinary characters into Brauer characters. In this case the necessary number depth of iterations can be computed; the `i`-th entry of the returned list is `fail` if there exists no nonnegative integral solution of the system ``x`*` A`=` B`[i]`, and it is the solution otherwise.

  Note that the result is a list of `fail` if A has not full rank, even if there might be a unique integral solution for some equation system.

- `LinearIndependentColumns(` mat`) F`

  Called with a matrix mat, `LinearIndependentColumns` returns a maximal list of column positions such that the restriction of mat to these columns has the same rank as mat.

- `PadicCoefficients(` A`,` Amodpinv`,` b`,` prime`,` depth`) F`

  Let A be an integral matrix, prime a prime integer, Amodpinv an inverse of A modulo prime, b an integral vector, and depth a nonnegative integer. `PadicCoefficients` returns the list [ x0, x1, ¼, xl, bl+1 ] describing the prime-adic approximation of b (see above), where l = depth or l is minimal with the property that bl+1 = 0.

- `IntegralizedMat(` A`) F`
- `IntegralizedMat(` A`,` inforec`) F`

  `IntegralizedMat` returns for a matrix A of cyclotomics a record intmat with components `mat` and `inforec`. Each family of algebraic conjugate columns of A is encoded in a set of columns of the rational matrix ``intmat`.mat` by replacing cyclotomics in A by their coefficients w.r.t. an integral basis. ``intmat`.inforec` is a record containing the information how to encode the columns.

  If the only argument is A, the value of the component `inforec` is computed that can be entered as second argument inforec in a later call of `IntegralizedMat` with a matrix B that shall be encoded compatibly with A.

- `DecompositionInt(` A`,` B`,` depth`) F`

  `DecompositionInt` does the same as `Decomposition` (see Decomposition), except that A and B must be integral matrices, and depth must be a nonnegative integer.

  ## 25.3 Lattice Reduction

- `LLLReducedBasis( [`L`, ]`vectors`[,` y`][, "linearcomb"][,` lllout`] ) F`

  provides an implementation of the LLL algorithm by Lenstra, Lenstra and Lovász (see LLL82, Poh87). The implementation follows the description on pages 94f. in Coh93.

  `LLLReducedBasis` returns a record whose component `basis` is a list of LLL reduced linearly independent vectors spanning the same lattice as the list vectors. L must be a lattice, with scalar product of the vectors v and w given by `ScalarProduct(` L`,` v`,` w`)`. If no lattice is specified then the scalar product of vectors given by `ScalarProduct(` v`,` w`)` is used.

  In the case of the option `"linearcomb"`, the result record contains also the components `relations` and `transformation`, with the following meaning. `relations` is a basis of the relation space of vectors, i.e., of vectors x such that ``x`*` vectors`` is zero. `transformation` gives the expression of the new lattice basis in terms of the old, i.e., `transformation *` vectors`` equals the `basis` component of the result.

  Another optional argument is y, the ``sensitivity'' of the algorithm, a rational number between [1/4] and 1 (the default value is [3/4]).

  The optional argument lllout is a record with the components `mue` and `B`, both lists of length k, with the meaning that if lllout is present then the first k vectors in vectors form an LLL reduced basis of the lattice they generate, and ``lllout`.mue` and ``lllout`.B` contain their scalar products and norms used internally in the algorithm, which are also present in the output of `LLLReducedBasis`. So lllout can be used for ``incremental'' calls of `LLLReducedBasis`.

  The function `LLLReducedGramMat` (see LLLReducedGramMat) computes an LLL reduced Gram matrix.

      gap> vectors:= [ [ 9, 1, 0, -1, -1 ], [ 15, -1, 0, 0, 0 ],
      >                [ 16, 0, 1, 1, 1 ], [ 20, 0, -1, 0, 0 ],
      >                [ 25, 1, 1, 0, 0 ] ];;
      gap> LLLReducedBasis( vectors, "linearcomb" );; Display( last );
      rec(
        basis := [ [ 1, 1, 1, 1, 1 ], [ 1, 1, -2, 1, 1 ], [ -1, 3, -1, -1, -1 ],
            [ -3, 1, 0, 2, 2 ] ],
        relations := [ [ -1, 0, -1, 0, 1 ] ],
        transformation :=
         [ [ 0, -1, 1, 0, 0 ], [ -1, -2, 0, 2, 0 ], [ 1, -2, 0, 1, 0 ],
            [ -1, -2, 1, 1, 0 ] ],
        mue := [ [  ], [ 2/5 ], [ -1/5, 1/3 ], [ 2/5, 1/6, 1/6 ] ],
        B := [ 5, 36/5, 12, 50/3 ] )

- `LLLReducedGramMat(` G`) F`
- `LLLReducedGramMat(` G`,` y`) F`

  `LLLReducedGramMat` provides an implementation of the LLL algorithm by Lenstra, Lenstra and Lovász (see LLL82, Poh87). The implementation follows the description on pages 94f. in Coh93.

  Let G the Gram matrix of the vectors (b1, b2, ¼, bn); this means G is either a square symmetric matrix or lower triangular matrix (only the entries in the lower triangular half are used by the program).

  `LLLReducedGramMat` returns a record whose component `remainder` is the Gram matrix of the LLL reduced basis corresponding to (b1, b2, ¼, bn). If G is a lower triangular matrix then also the `remainder` component of the result record is a lower triangular matrix.

  The result record contains also the components `relations` and `transformation`, which have the following meaning.

  `relations` is a basis of the space of vectors (x1,x2,¼,xn) such that åi=1n xi bi is zero, and `transformation` gives the expression of the new lattice basis in terms of the old, i.e., `transformation` is the matrix T such that T ·G ·Ttr is the `remainder` component of the result.

  The optional argument y denotes the ``sensitivity'' of the algorithm, it must be a rational number between [1/4] and 1; the default value is y = [3/4].

  The function `LLLReducedBasis` (see LLLReducedBasis) computes an LLL reduced basis.

      gap> g:= [ [ 4, 6, 5, 2, 2 ], [ 6, 13, 7, 4, 4 ],
      >    [ 5, 7, 11, 2, 0 ], [ 2, 4, 2, 8, 4 ], [ 2, 4, 0, 4, 8 ] ];;
      gap> LLLReducedGramMat( g );; Display( last );
      rec(
        remainder := [ [ 4, 2, 1, 2, -1 ], [ 2, 5, 0, 2, 0 ], [ 1, 0, 5, 0, 2 ],
            [ 2, 2, 0, 8, 2 ], [ -1, 0, 2, 2, 7 ] ],
        relations := [  ],
        transformation :=
         [ [ 1, 0, 0, 0, 0 ], [ -1, 1, 0, 0, 0 ], [ -1, 0, 1, 0, 0 ],
            [ 0, 0, 0, 1, 0 ], [ -2, 0, 1, 0, 1 ] ],
        mue := [ [  ], [ 1/2 ], [ 1/4, -1/8 ], [ 1/2, 1/4, -2/25 ],
            [ -1/4, 1/8, 37/75, 8/21 ] ],
        B := [ 4, 4, 75/16, 168/25, 32/7 ] )

  ## 25.4 Orthogonal Embeddings

- `OrthogonalEmbeddings(` gram`[, "positive"][,` maxdim`] ) F`

  computes all possible orthogonal embeddings of a lattice given by its Gram matrix gram, which must be a regular matrix. In other words, all solutions X of the problem

  Xtr ·X = gram

  are calculated (see Ple90). Usually there are many solutions X but all their rows are chosen from a small set of vectors, so `OrthogonalEmbeddings` returns the solutions in an encoded form, namely as a record with components

  `vectors`
    the list L = [ x1, x2, ¼, xn ] of vectors that may be rows of a solution; these are exactly those vectors that fulfill the condition xi ·gram -1 ·xitr £ 1 (see ShortestVectors), and we have gram = åni=1 xitr ·xi,

  `norms`
    the list of values xi ·gram -1 ·xitr, and

  `solutions`
    a list S of lists; the i-th solution matrix is ``L`` S`[`i`]` , so the dimension of the i-th solution is the length of ``S`[`i`]`.

  The optional argument `"positive"` will cause `OrthogonalEmbeddings` to compute only vectors xi with nonnegative entries. In the context of characters this is allowed (and useful) if gram is the matrix of scalar products of ordinary characters.

  When `OrthogonalEmbeddings` is called with the optional argument maxdim (a positive integer), only solutions up to dimension maxdim are computed; this will accelerate the algorithm in some cases.

      gap> b:= [ [ 3, -1, -1 ], [ -1, 3, -1 ], [ -1, -1, 3 ] ];;
      gap> c:=OrthogonalEmbeddings( b );; Display( c );
      rec(
        vectors := [ [ -1, 1, 1 ], [ 1, -1, 1 ], [ -1, -1, 1 ], [ -1, 1, 0 ],
            [ -1, 0, 1 ], [ 1, 0, 0 ], [ 0, -1, 1 ], [ 0, 1, 0 ], [ 0, 0, 1 ] ],
        norms := [ 1, 1, 1, 1/2, 1/2, 1/2, 1/2, 1/2, 1/2 ],
        solutions := [ [ 1, 2, 3 ], [ 1, 6, 6, 7, 7 ], [ 2, 5, 5, 8, 8 ],
            [ 3, 4, 4, 9, 9 ], [ 4, 5, 6, 7, 8, 9 ] ] )
      gap> c.vectors{ c.solutions[1] };
      [ [ -1, 1, 1 ], [ 1, -1, 1 ], [ -1, -1, 1 ] ]

  gram may be the matrix of scalar products of some virtual characters. From the characters and the embedding given by the matrix X, `Decreased` (see Decreased) may be able to compute irreducibles, see Reducing Virtual Characters.

- `ShortestVectors(` G`,` m`[, "positive"] ) F`

  Let G be a regular matrix of a symmetric bilinear form, and m a nonnegative integer. `ShortestVectors` computes the vectors x that satisfy x ·G ·xtr £ m , and returns a record describing these vectors. The result record has the components

  `vectors`
    list of the nonzero vectors x, but only one of each pair (x,-x),

  `norms`
    list of norms of the vectors according to the Gram matrix G.
  If the optional argument `"positive"` is entered, only those vectors x with nonnegative entries are computed.

      gap> g:= [ [ 2, 1, 1 ], [ 1, 2, 1 ], [ 1, 1, 2 ] ];;
      gap> ShortestVectors(g,4);; Display( last );
      rec(
        vectors := [ [ -1, 1, 1 ], [ 0, 0, 1 ], [ -1, 0, 1 ], [ 1, -1, 1 ],
            [ 0, -1, 1 ], [ -1, -1, 1 ], [ 0, 1, 0 ], [ -1, 1, 0 ], [ 1, 0, 0 ] ],
        norms := [ 4, 2, 2, 4, 2, 4, 2, 2, 2 ] )

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  GAP 4 manual
  May 2002