K-Theory and Formality

We compare several notions of equivariant formality for complex K-theory with respect to a compact Lie group action, including surjectivity of the forgetful map and the weak equivariant formality of Harada-Landweber, and find all are equivalent under standard hypotheses. As a consequence, we present an expression for the equivariant K-theory of the isotropy action of H on a homogeneous space G/H in all the classical cases. The proofs involve mainly homological algebra and arguments with the Atiyah-Hirzebruch-Leray-Serre spectral sequence, but a more general result depends on a map of spectral sequences from Hodgkin's Kunneth spectral sequence in equivariant K-theory to that in Borel cohomology that seems not to have been otherwise defined. The hypotheses for the main structure result are analogous to a previously announced characterization of cohomological equivariant formality, first proved here, expanding on the results of Shiga and Takahashi.