Fourth derivative
= Fourth derivative
{title2=$f^{(4)}=\dfrac{d}{dx}f'''$}
The <fourth derivative> is the <derivative> of the <third derivative>. It vanishes for a <polynomial> of <polynomial degree> at most three. For a <cubic spline>, this statement applies inside each knot <interval>; a jump of the <third derivative> can prevent a classical <fourth derivative> from existing at a <spline knot>.