void convolve(double *a, int *na, double *b, int *nb, double *ab)
{
    int i, j, nab = *na + *nb - 1;

    for(i = 0; i < nab; i++)
        ab[i] = 0.0;
    for(i = 0; i < *na; i++)
        for(j = 0; j < *nb; j++)
            ab[i + j] += a[i] * b[j];
}