A matrix B is in Smith normal form if the only non-zero entries are on the diagonal (for non-square matrices, this simply means that bij=0 for i≠j) and bi,i divides bi+1,i+1. The elements bi,i are called invariant factors and are used to describe the structure of finite abelian groups.
For any matrix A with coefficients in ℤ, there exist matrices U and V, invertible in ℤ, such that B=U A V is in Smith normal form and has coefficients in ℤ. The ismith command finds the matrices U, B and V.
A:=[[9,-36,30],[-36,192,-180],[30,-180,180]]:; U,B,V:=ismith(A) |
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The invariant factors are 3, 12 and 60.