1 Introduction -- 1.1 Introduction -- 1.2 Wide popularity of the SABR -- 1.3 Simple derivation -- 1.4 Modifications and extensions of the SABR -- 1.5 CMS and the SABR -- 1.6 Approximation accuracy and its improvements -- 1.7 About this book -- 2 Exact Solutions to CEV Model with Stochastic Volatility -- 2.1 Introduction -- 2.2 Transforming CEV Process into the Bessel One -- 2.3 Solution behavior near singular point x = 0, integrability, flux -- 2.4 Laplace Transform -- 2.5 Probability distributions -- 2.6 Back to CEV model -- 2.6.1 Option pricing through Chi Square distributions -- 2.7 Alternative expressions for CEV option values -- 2.8 CEV Model with Stochastic Volatility -- 2.9 Conclusion -- 3 Classic SABR Model: Exactly Solvable Cases -- 3.1 Introduction -- 3.2 Probability Density Functions for the Free Normal and Log-Normal SABR, Probabilistic Approach -- 3.3 Deriving PDFs using Kolmogorov equations -- 3.4 Option Value for the Free Normal SABR -- 3.5 Option Value for the Lognormal SABR -- 3.6 The Zero Correlation case -- 4 Classic SABR Model: Heat Kernel Expansion and Projection on Solvable Models -- 4.1 Introduction -- 4.2 Invariant forms of Diffusion Equations -- 4.3 Heat Kernel Expansion -- 4.4 Non-Zero Correlation General Case -- 4.5 Conclusion -- References.