error in BSpline

[morder]

sometimes derivativeBSpline function returns wrong values
the right code is

int n = myX.length;
double[] newControlPointsX = new double[n - 1];
double[] newControlPointsY = new double[n - 1];

for (int i = 0; i < n - 1; i++) {
    newControlPointsX[i] = degree * (myX[i + 1] - myX[i]) / (knots[i + degree + 1] - knots[i + 1]);
    newControlPointsY[i] = degree * (myY[i + 1] - myY[i]) / (knots[i + degree + 1] - knots[i + 1]);
}

double[] newKnots = new double[knots.length - 2];
System.arraycopy(knots, 1, newKnots, 0, newKnots.length);

return new BSpline(newControlPointsX, newControlPointsY, newKnots);