Roberto Volpato

Mathieu twining characters for K3

The analogue of the McKay-Thompson series for the proposed Mathieu group action on the elliptic genus of K3 is analysed. The corresponding NS-sector twining characters have good modular properties and satisfy remarkable replication identities. These observations provide strong support for the conjecture that the elliptic genus of K3 carries indeed an action of the Mathieu group

Mathieu Moonshine in the elliptic genus of K3

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The Gravity Dual of the Ising Model

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Getting superstring amplitudes by degenerating Riemann surfaces

It has recently been conjectured that the elliptic genus of K3 can be written in terms of dimensions of Mathieu group

A K3 sigma model with Z_2^8:M_20 symmetry

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Linear relations among holomorphic quadratic differentials and induced Siegel's metric on Mg

representations. Some further evidence for this idea was subsequently found by studying the twining genera that are obtained from the elliptic genus upon replacing dimensions of Mathieu group representations by their characters. In this paper we find explicit formulae for all (remaining) twining genera by making an educated guess for their general modular properties. This allows us to identify the decomposition of all expansion coefficients in terms of dimensions of

On symmetries of N = (4, 4) sigma models on T4

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