A bell-shaped or $B$-spline $u=\beta(x)$ interpolates the data
$$
\beta(-2)=0, \quad \beta(-1)=1, \quad \beta(0)=4, \quad \beta(1)=1, \quad \beta(2)=0 .
$$
(a) Find the explicit formula for the natural $B$-spline and plot its graph. (b) Show that $\beta(x)$ also satisfies the homogeneous clamped boundary conditions $u^{\prime}(-2)=u^{\prime}(2)=0$.
(c) Show that $\beta(x)$ also satisfies the periodic boundary conditions. Thus, for this particular interpolation problem, the natural, clamped, and periodic splines happen to coincide.
(d) Show that $\beta^{\star}(x)=\left\{\begin{array}{ll}\beta(x), & -2 \leq x \leq 2 \\ 0, & \text { otherwise, }\end{array}\right.$ defines a $\mathrm{C}^2$ spline on every interval $[-k, k]$.