The Laplace transform and polynomial approximation in L2

This short note gives a sufficient condition for having the class of polynomials dense in the space of square integrable functions with respect to a finite measure dominated by the Lebesgue measure in the real line, here denoted by $L^2$. It is shown that if the Laplace transform of the measure in play is bounded in a neighbourhood of the origin, then the moments of all order are finite and the class of polynomials is dense in $L^2$. The existence of the moments of all orders is well known for the case where the measure is concentrated in the positive real line (see Feller, 1966), but the result concerning the polynomial approximation is original, even thought the proof is relatively simple. Additionally, an alternative stronger condition easier to be verified not involving the calculation of the Laplace transform is given. The condition essentially says that the density of the measure should have exponential decaying tails. The tools presented are of interest for constructing semiparametric extensions of classic parametric models.