## Problem 31

Posted 11/05/2016
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This problem is about a special class of $2$-column $n$-row arrays.

Suppose $F$ is an $n\times 2$ array with fillings of $\{1,2,3,\cdots,2n\}$ such that elements are increasing from bottom to top in each column. Clearly, there are $\binom{2n}{n}$ such arrays. We let $F(i,j)$ denote the element in the $i$-th row and $j$-th column, counting from bottom to top and left to right. If $F(i+1,1)-F(i,1)=1$ or $F(i+1,2)-F(i,2)=1$ for any $1\leq i \leq n-1$, we say $F$ is a degenerate array. This type of arrays were introduced by Harmse and Remmel in this paper.

For example, there are ten $3\times 2$ degenerate arrays, pictured as follows.

Suppose we let $a_n$ denote the number of $n\times 2$ degenerate array.

Find $a_n$.