Exact Lebesgue Constants for Interpolatory $\mathscr L$-Splines of Third Order

In this paper, we obtain the Lebesgue constants for interpolatory $\mathscr L$-splines of third order with uniform nodes, i.e., the norms of interpolation operators from $\mathrm C$ to $\mathrm C$ describing the process of interpolation of continuous bounded and continuous periodic functions by $\mathscr L$-splines of third order with uniform nodes on the real line. As a corollary, we obtain exact Lebesgue constants for interpolatory polynomial parabolic splines with uniform nodes.