double der_p( int degree, int l, double* u, double* pi, int i )
{
	if ( l == 0 )
		return pi[i];
	else if ( l > 0 ) {
		double	a = degree - l;
		double	b = u[i+degree-l] - u[i];
		return a / b * ( der_p(degree, l-1, u, pi, i) - der_p(degree, l-1, u, pi, i-1) );
	}
}

double der( int degree, int l, double t, double* u, double* pi, int i )
{
	double	sum = 0.0;

	for ( int j = i-degree+l+1; j <= i; j++ )
		sum += N_v(j, degree-l, t, u) * der_p(degree, l, u, pi, j);

	return sum;
}