Relating smoothness to expressions involving Fourier coefficients or to a Fourier transform

Coefficients of expansion of a function by trigonometric, algebraic and spherical harmonic orthogonal polynomials are related to the smoothness of that function. Hausdorff-Young type and Hardy-Littlewood type inequalities will be utilized. Expressions involving the Fourier transform of a function are also related to the measure of smoothness of that function.