Fast Decreasing Trigonometric Polynomials and Applications

We construct trigonometric polynomials that fast decrease towards ± π ± π . We apply them to construct a trigonometric polynomial the derivative of which interpolates the derivative of a given 2π 2 π -periodic function, at some prescribed distinct points in [-π ,π ) [ - π , π ) , and vanishes at some other prescribed points in that interval. The construction requires that the function possesses derivatives where the interpolation is supposed to take place. Still, we are able to apply the result to trigonometric approximation of a 2π 2 π -periodic piecewise algebraic polynomial which is merely continuous, while interpolating its derivative at some points (that, obviously, are not knots).