# 65.25 CheckMat

`CheckMat( C )`

`CheckMat` returns a parity check matrix of C. The code consists of all words orthogonal to each of the rows of this matrix. The transpose of the matrix is a right inverse of the generator matrix. The parity check matrix is computed from either the generator matrix, the generator polynomial, the check polynomial or the elements of C (if possible), whichever is available.

If C is a non-linear code, the function returns an error.

```    gap> CheckMat( HammingCode(3, GF(2) ) );
[ [ 0*Z(2), 0*Z(2), 0*Z(2), Z(2)^0, Z(2)^0, Z(2)^0, Z(2)^0 ],
[ 0*Z(2), Z(2)^0, Z(2)^0, 0*Z(2), 0*Z(2), Z(2)^0, Z(2)^0 ],
[ Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0, 0*Z(2), Z(2)^0 ] ]
gap> CheckMat( RepetitionCode( 5, GF(25) ) );
[ [ Z(5)^0, Z(5)^2, 0*Z(5), 0*Z(5), 0*Z(5) ],
[ 0*Z(5), Z(5)^0, Z(5)^2, 0*Z(5), 0*Z(5) ],
[ 0*Z(5), 0*Z(5), Z(5)^0, Z(5)^2, 0*Z(5) ],
[ 0*Z(5), 0*Z(5), 0*Z(5), Z(5)^0, Z(5)^2 ] ]
gap> CheckMat( WholeSpaceCode( 12, GF(4) ) );
[  ] ```

GAP 3.4.4
April 1997