Past exam of the mathematics course of the University of Cambridge / 2026 / ii / Paper 3 / 3H / Solution

A cyclic code is a linear subspace invariant under cyclic coordinate shift. Identifying words with ${\mathbb{F}}_2[X]/(X^n-1)$ makes such codes exactly ideals. Since this quotient is a principal ideal ring, every code is generated uniquely by a monic divisor of $X^n-1$. For $n=7$,so its three distinct irreducible factors have $2^3=8$ monic divisors and there are eight cyclic codes.