Past exam of the numerical analysis course of the University of Oxford / 2015 / 2 / a

The polynomial $p_n$ interpolates $f$ at the interpolation nodes whenFor distinct interpolation nodes, define the Lagrange cardinal polynomialsThe denominators are nonzero, each polynomial degree is at most $n$, and ${\ell }_i(x_j)={\delta }_{ij}$, with ${\delta }_{ij}$ the Kronecker delta. Thereforeis a Lagrange interpolation polynomial belonging to the vector space of univariate polynomials ${\Pi }_n$, and it has all the required sampled values.

If $r_n\in {\Pi }_n$ also works, $p_n-r_n$ has $n+1$ distinct roots of a polynomial while its polynomial degree is at most $n$. The root bound for a polynomial forces it to be the zero polynomial, proving uniqueness. The phrase “degree $n$” denotes the allowed maximum; the actual polynomial degree may be smaller. No continuity assumption is needed merely to construct the interpolation polynomial from these finitely many values.