Two congruences involving harmonic numbers with applications

Abstract: The harmonic numbers $H_n=\sum_{0<k\le n}1/k\ (n=0,1,2,\ldots)$ play important roles in mathematics. Let $p\>3$ be a prime. With helps of some combinatorial identities, we establish the following two new congruences: $$\sum_{k=1}^{p-1}\frac{\binom{2k}k}kH_k\equiv\frac13\left(\frac p3\right)B_{p-2}\left(\frac13\right)\pmod{p}$$ and $$\sum_{k=1}^{p-1}\frac{\binom{2k}k}kH_{2k}\equiv\frac7{12}\left(\frac p3\right)B_{p-2}\left(\frac13\right)\pmod{p},$$ where $B_n(x)$ denotes the Bernoulli polynomial of degree $n$. As an application, we determine $\sum_{n=1}^{p-1}g_n$ and $\sum_{n=1}^{p-1}h_n$ modulo $p^3$, where $$g_n=\sum_{k=0}^n\binom nk^2\binom{2k}k\quad\mbox{and}\quad h_n=\sum_{k=0}^n\binom nk^2C_k$$ with $C_k=\binom{2k}k/(k+1)$.