Lam and Pylyavskyy introduced loop symmetric functions as a generalization of symmetric functions. They defined loop Schur functions as generating functions over semistandard tableaux with respect to a "colored weight," and they proved a Jacobi-Trudi-style determinantal formula for these generating functions. We prove that loop Schur functions can be expressed as a ratio of "loop alternants," extending the analogy with Schur functions. As an application, we give a new proof of the loop version of the Murnaghan-Nakayama rule.other words, the action tropicalizes to a piecewise-linear formula for the combinatorial Rmatrix for tensor products of affine crystals-in this case, the one-row Kirillov-Reshetikhin crystals of type A (1) n−1 [HHI + 01]. This suggests that a function on tensor products of one-row crystals which is invariant under the combinatorial R-matrix should be the tropicalization of a ratio of loop symmetric functions. Lam and Pylyavskyy showed that the intrinsic energy, an important function in affine crystal theory, is in fact the tropicalization of a certain loop Schur function [LP13b] (this loop Schur function turns out to be related to our alternant formulas; see Remark 3.6). Additionally, tensor products of one-row crystals can be viewed as states in the (generalized) Box-Ball system, a well-studied cellular automaton that exhibits soliton behavior [HHI + 01]. Formulas for the scattering of a given state into solitons are conjecturally given by tropicalizations of a cylindric variant of loop Schur functions [LPS].We note that the birational S m action also arose in the context of the local Langlands program [BK00b], and was studied in [Eti03]. Loop symmetric functions have also found application in Gromov-Witten/Donaldson-Thomas theory [RZ13]; this was Ross' motivation for proving the loop Murnaghan-Nakayama rule [Ros14].This paper is organized as follows. In Section 2, we review the basics of loop symmetric functions and the birational S m action. Section 3 contains statements and proofs of the two ratio of alternants formulas (Theorems 3.1 and 3.3), and Section 4 discusses the loop Murnaghan-Nakayama rule (Theorem 4.1).