Radford's S^4 formula for co-Frobenius Hopf algebras
  This note extends Radford's formula for the fourth power of the antipode of a finite dimensional Hopf algebra to co-Frobenius Hopf algebras and studies equivalent conditions to a Hopf algebra being involutory for finite dimensional and co-Frobenius Hopf algebras. 
Margaret Beattie, Daniel Bulacu, Blas Torrecillas