Perfect spline
In the mathematical subfields function theory and numerical analysis, a univariate polynomial spline of order is called a perfect spline if its -th derivative is equal to or between knots and changes its sign at every knot.
The term was coined by Isaac [Jacob Schoenberg].
Perfect splines often give solutions to various extremal problems in mathematics. For example, norms of periodic perfect splines are equal to Favard's constants.