Short basis of a lattice : lll

### 5.50.7 Short basis of a lattice : lll

lll takes as argument an invertible matrix M with integer coefficients.
lll returns (S,A,L,O) such that:

- the rows of S is a short basis of the ℤ-module generated by the rows of M,
- A is the change-of-basis matrix from the short basis to the basis defined by the rows of M (A*M=S),
- L is a lower triangular matrix, the modulus of its non diagonal coefficients are less than 1/2,
- O is a matrix with orthogonal rows such that L*O=S.

Input :

(S,A,L,O):=lll(M:=[[2,1],[1,2]])

Output :

[[-1,1],[2,1]], [[-1,1],[1,0]], [[1,0],[1/-2,1]], [[-1,1],[3/2,3/2]]

Hence :
S=[[-1,1],[2,1]]
A=[[-1,1],[1,0]]
L=[[1,0],[1/-2,1]]
O=[[-1,1],[3/2,3/2]]
Hence the original basis is v1=[2,1], v2=[1,2]
and the short basis is w1=[-1,1], w2=[2,1].
Since w1=-v1+v2 and w2=v1 then :
A:=[[-1,1],[1,0]], A*M==S and L*O==S.
Input :

(S,A,L,O):=lll([[3,2,1],[1,2,3],[2,3,1]])

Output :

S=[[-1,1,0],[-1,-1,2],[3,2,1]]
A= [[-1,0,1],[0,1,-1],[1,0,0]]
L= [[1,0,0],[0,1,0],[(-1)/2,(-1)/2,1]]
O= [[-1,1,0],[-1,-1,2],[2,2,2]]

Input :
M:=[[3,2,1],[1,2,3],[2,3,1]]
Properties :
A*M==S and L*O==S