Superconformal Algebras and Mock Theta Functions 2. Rademacher Expansion for K3 Surface

The elliptic genera of the K3 surfaces, both compact and non-compact cases, are studied by using the theory of mock theta functions. We decompose the elliptic genus in terms of the N=4 superconformal characters at level-1, and present an exact formula for the coefficients of the massive (non-BPS) representations using the Poincare-Maass series.