A natural spline with \(n=3\) intervals has \(4\cdot3=12\) unknowns. How many interpolation and continuity equations are there in total, not counting the boundary ones?
A cubic spline \(S\) is a piecewise function, each piece a polynomial of degree three, that interpolates the data \((x_i,y_i)\) and is of class \(C^2\) at the nodes.