  The AGT relations allow one to convert the Zamolodchikov asymptotic formula
for the conformal block into the instanton expansion of the Seiberg-Witten
prepotential for theory with two colors and four fundamental flavors. This
provides an explicit formula for the instanton corrections in this model,
resolving in this way an old problem in Seiberg-Witten theory. The answer is
especially elegant for vanishing matter masses, then the bare charge of gauge
theory 16q_0 = 16e^{i\pi\tau_0} plays the role of a branch point on the
spectral torus. The exact prepotential at this point is F a^2\log q with q\neq
q_0, unlike the case of another conformal theory, with massless adjoint field.
Instead, 16q_0 = \theta_{10}^4/\theta_{00}^4(q) = 16q(1+O(q)), i.e. the
Zamolodchikov asymptotics gives rise, in particular, to an exact
non-perturbative beta-function so that the effective coupling differs from the
bare charge by infinite number of instantonic corrections.
