A multiplicative comparison of Waldhausen and Segal K-theory

In this paper, we establish a multiplicative equivalence between two multiplicative algebraic $K$-theory constructions, Elmendorf and Mandell's version of Segal's $K$-theory and Blumberg and Mandell's version of Waldhausen's $S_\bullet$ construction. This equivalence implies that the ring spectra, algebra spectra, and module spectra constructed via these two classical algebraic $K$-theory functors are equivalent as ring, algebra or module spectra, respectively. It also allows for comparisions of spectrally enriched categories constructed via these definitions of $K$-theory. As both the Elmendorf--Mandell and Blumberg--Mandell multiplicative versions of $K$-theory encode their multiplicativity in the language of multicategories, our main theorem is that there is multinatural transformation relating these two symmetric multifunctors that lifts the classical functor from Segal's to Waldhausen's construction. Along the way, we provide a slight generalization of the Elmendorf--Mandell construction to symmetric monoidal categories.