THH(R) congruent with R circle times operator S1 for E∞ ring spectra Original Research Article

We prove that the topological Hochschild homology spectrum THH(R) of an E∞ spectrum R is the S1-indexed sum of copies R in the category of E∞, ring spectra. As a consequence, we obtain a natural S1-action and compatible power operations on THH(R). In addition, THH(R) admits an A∞-comultiplication making it an A∞ Hopf algebra spectrum over R.