Let $G=SU(2)$ and $\Omega G$ the space of based loops in SU(2). Motivated by
the theory of Hamiltonian $LG$-spaces, we explicitly compute the topological
equivariant $K$-theory $K_G^*(\Omega G)$ as an $R(G)$-module.
Peterson varieties are a special class of Hessenberg varieties that have been
extensively studied e.g. by Peterson, Kostant, and Rietsch, in connection with
the quantum cohomology of the flag variety. In this manuscript, we develop a
generalized Schubert calculus, and in particular a positive Chevalley-Monk
formula, for the ordinary and Borel-equivariant cohomology of the Peterson
variety $Y$ in type $A_{n-1}$, with respect to a natural $S^1$-action arising
from the standard action of the maximal torus on flag varieties.