you need both square integrable and smooth both is provided by the condition band limited and no you can't sample a sinusoid that is of course you can sample it but the reconstruction formula will not converge since the reconstruction formula is essentially a representation in an orthonormal basis it converges in the l sense exactly if the coefficient sequence is square summable and the sample sequence of a sinusoid is not square summable because of its periodicity but that was already discussed ad nauseatum years ago so consult the archive pages to find the points of view of the participants