Perturbation Theory in Angular Quantization Approach and the Expectation Values of Exponential Fields in Sin-Gordon Model

In angular quantization approach a perturbation theory for the Massive Thirring Model (MTM) is developed, which allows us to calculate Vacuum Expectation Values of exponential fields in sin-Gordon theory near the free fermion point in first order of MTM coupling constant $g$. The Hankel-transforms play an important role when carrying out this calculations. The expression we have found coincides with that of the direct expansion over $g$ of the exact formula conjectured by S.Lukyanov and A.Zamolodchikov.