Summary. Let 5ek be the k-dimensional subspace spanned by the translates (a(.-2rtj/k), j=0, 1, ...,k-l, of a continuous, piecewise smooth, complexvalued, 2 ~-periodic function qS. For a given functionfe L z (--rt, n), its least squares approximant Szf from ~ can be expressed in terms of an orthonormal basis.If f is continuous, Skfcan be computed via its discrete analogue by fast Fourier transform. The discrete least squares approximant is used to approximate Fourier coefficients, and this complements the works of Gautschi on attenuation factors. Examples of 5~k include the space of trigonometric polynomials where ~b is the de la Valle6 Poussin kernel, algebraic polynomial splines where ~b is the periodic B-spline, and trigonometric polynomial splines where ~b is the trigonometric B-spline. (1991) : 41A 15, 42A 16, 65D07
Mathematics Subject Classification