1. Extensions of Hopf algebras;Andruskiewitsch;Algebra i Analiz,1995
2. Normal Hopf subalgebras of semisimple Hopf algebras;Burciu;Proc. Amer. Math. Soc.,2009
3. On higher Frobenius–Schur indicators;Kashina;Mem. Amer. Math. Soc.,2006
4. Characters of Hopf algebras;Larson;J. Algebra,1971