Computational complexity of the word problem in modal algebras

The paper deals with the word problem for modal algebras. It is proved that, for the variety of all modal algebras, the word problem is $\mathrm{PSPACE}$-complete if only constant modal terms or only $0$-generated modal algebras are considered. We also consider the word problem for different varieties of modal algebras. It is proved that the word problem for a variety of modal algebras can be $C$-hard, for any complexity class or unsolvability degree $C$ containing a $C$-complete problem. It is shown how to construct such varieties.