2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/139340Let $K$ be an abelian extension of the rationals. Let $S(K)$ be the Schur group of $K$ and let $CC(K)$ be the subgroup of $S(K)$ generated by classes containing cyclic cyclotomic algebras. We characterize when $CC(K)$ has finite index in $S(K)$ in terms of the relative position of $K$ in the lattice of cyclotomic extensions of the rationals.12 pagesRepresentation TheoryRings and Algebras16K50; 12G05; 16S35text