A simple discharging method for forbidden subposet problems

Abstract

The poset $Y_{k+1, 2}$ consists of $k+2$ distinct elements $x_1$, $x_2$, \dots, $x_{k}$, $y_1$,$y_2$, such that $x_1 \le x_2 \le \dots \le x_{k} \le y_1$,~$y_2$. The poset $Y'_{k+1, 2}$ is the dual of $Y_{k+1, 2}$ Let $\rm{La}^{\sharp}(n,\{Y_{k+1, 2}, Y'_{k+1, 2}\})$ be the size of the largest family $\mathcal{F} \subset 2^{[n]}$ that contains neither $Y_{k+1,2}$ nor $Y'_{k+1,2}$ as an induced subposet. Methuku and Tompkins proved that $\rm{La}^{\sharp}(n, \{Y_{3,2}, Y'_{3,2}\}) = \Sigma(n,2)$ for $n \ge 3$ and they conjectured the generalization that if $k \ge 2$ is an integer and $n \ge k+1$, then $\rm{La}^{\sharp}(n, \{Y_{k+1,2}, Y'_{k+1,2}\}) = \Sigma(n,k)$. In this paper, we introduce a simple discharging approach and prove this conjecture.