 Solution

With $K\ge 2$ distinct knots ${\kappa }_1<\cdots <{\kappa }_K$, a natural cubic spline is a $C^2$ function on the real line, cubic on each interval between consecutive knots and affine outside the two outer knots. The latter condition gives $m^{\prime \prime }({\kappa }_1)=m^{\prime \prime }({\kappa }_K)=0$.

There are $4(K-1)$ coefficients for the interior cubic pieces. Matching values and the first two derivatives at the $K-2$ interior knots imposes $3(K-2)$ independent constraints, and the two natural endpoint conditions impose two more. The affine continuations are then determined by endpoint values and slopes. The dimension of a vector space is thereforeOne explicit basis uses $1,x$ and $d_j(x)-d_{K-1}(x)$ for $j=1,\dots ,K-2$, whereThe cubic and quadratic terms cancel in each difference above ${\kappa }_K$, making it affine there; below ${\kappa }_1$ the truncated powers vanish. The changes in third derivative at individual knots show independence of these basis functions. This count uses the linear independence of the spline constraints and counts the listed outer knots as boundary knots. If a convention instead lists $K$ interior knots and two additional boundaries, the dimension is $K+2$. With only one listed knot and linearity on both sides, the $C^2$ condition forces a globally affine function, so the exceptional dimension is two.