where η η ( q ) is the Dedekind eta function, and θ θ i ( 0 | q ) are theta functions of the nome q = e 2 π π i τ τ , for example θ θ 3 ( 0 | q ) = ∑ ∑ n ∈ ∈ Z q n 2 2 . The modular S-matrix, i.e. the matrix S such that χ χ i ( − − 1 τ τ ) = ∑ ∑ j S i j χ χ j ( τ τ ) , is