MathDB
2023 Putnam B4

Source:

December 3, 2023
PutnamPutnam 2023

Problem Statement

For a nonnegative integer nn and a strictly increasing sequence of real numbers t0,t1,,tnt_0, t_1, \ldots, t_n, let f(t)f(t) be the corresponding real-valued function defined for tt0t \geq t_0 by the following properties: (a) f(t)f(t) is continuous for tt0t \geq t_0, and is twice differentiable for all t>t0t>t_0 other than t1,,tnt_1, \ldots, t_n; (b) f(t0)=1/2f\left(t_0\right)=1 / 2; (c) limttk+f(t)=0\lim _{t \rightarrow t_k^{+}} f^{\prime}(t)=0 for 0kn0 \leq k \leq n; (d) For 0kn10 \leq k \leq n-1, we have f(t)=k+1f^{\prime \prime}(t)=k+1 when tk<t<tk+1t_k<t<t_{k+1}, and f(t)=n+1f^{\prime \prime}(t)=n+1 when t>tnt>t_n. Considering all choices of nn and t0,t1,,tnt_0, t_1, \ldots, t_n such that tktk1+1t_k \geq t_{k-1}+1 for 1kn1 \leq k \leq n, what is the least possible value of TT for which f(t0+T)=2023f\left(t_0+T\right)=2023?