SIAM JOURNAL ON DISCRETE MATHEMATICS, cilt.31, sa.2, ss.1454-1475, 2017 (SCI-Expanded)
We completely characterize possible indices of quasi-cyclic subcodes in a cyclic code for a very broad class of cyclic codes. We present enumeration results for quasi-cyclic subcodes of a fixed index and show that the problem of enumeration is equivalent to enumeration of certain vector subspaces in finite fields. In particular, we present enumeration results for quasi-cyclic subcodes of the simplex code and duals of certain Bose-Chaudhuri-Hocquenqhem codes. Our results are based on the trace representation of cyclic codes.