A class of constacyclic codes

A class of q^m-ary constacyclic codes is defined which has the property that the q-ary image is equivalent to a class of shortened cyclic codes. This description leads to the construction of nonlinear codes. As an example one can construct nonlinear codes of length N=q^m+1 over the field GF(q^m´), m´⩽m, with a minimum distance greater than N(1-R)m´/m, where R is the rate. For higher rates these codes are the best known. The codes are easily encoded and decoded