= Solution
Write the interpolating spline as
$$
P_{\mathbf x}f=\sum_{j=1}^nc_jN_j.
$$
The interpolation equations are $A_{\mathbf x}c=f_{\mathbf x}$, where $(f_{\mathbf x})_i=f(x_i)$, so
$$
\|c\|_{\ell_\infty}
\leq\|A_{\mathbf x}^{-1}\|_{\ell_\infty}
\|f_{\mathbf x}\|_{\ell_\infty}
\leq\|A_{\mathbf x}^{-1}\|_{\ell_\infty}\|f\|_\infty.
$$
The <B-splines> are nonnegative and form a <partition of unity> on the spline interval. Consequently
$$
|P_{\mathbf x}f(t)|
\leq\sum_j|c_j|N_j(t)
\leq\|c\|_{\ell_\infty}.
$$
Taking the supremum over $t$ and then over $\|f\|_\infty\leq1$ proves the <operator norm> bound
$$
\boxed{\|P_{\mathbf x}\|_{L_\infty}
\leq\|A_{\mathbf x}^{-1}\|_{\ell_\infty}}.
$$
Back to article page
