We show that the topological Hochschild homology of an –ring spectrum is an –ring spectrum. The proof is based on the fact that the tensor product of the operad for monoid structures and the little –cubes operad is an –operad, a result which is of independent interest.
"On the multiplicative structure of topological Hochschild homology." Algebr. Geom. Topol. 7 (4) 1633 - 1650, 2007. https://doi.org/10.2140/agt.2007.7.1633