AIME 2026a Problem 15

Let $a, b,$ and $n$ be positive integers with both $a$ and $b$ greater than or equal to $2$ and less than or equal to $2n.{}$ Define an $a \times b$ cell loop in a $2n \times 2n$ grid of cells to be the $2a + 2b - 4$ cells that surround an $(a - 2) \times (b - 2)$ (possibly empty) rectangle of cells in the grid. For example, the following diagram shows a way to partition a $6 \times 6$ grid of cells into $4$ cell loops. [asy] usepackage("mathptmx"); size(6cm); dotfactor = 30; fill((0, 2)--(0, 6)--(4, 6)--(4, 2)--cycle, royalblue); fill((1, 3)--(1, 5)--(3, 5)--(3, 3)--cycle, lightred); fill((4, 0)--(4, 6)--(6, 6)--(6, 0)--cycle, lightyellow); draw((1, 1)--(3, 1)^^(5, 1)--(5, 5)^^(1, 3)--(1, 5)--(3, 5)--(3, 3)--cycle^^(0, 0)--(0, 6)--(6, 6)--(6, 0)--cycle^^(0, 2)--(4, 2)^^(4, 0)--(4, 6), linewidth(4)); dot((2, 4)); for (int i = 1; i <= 5; i += 1) { draw((0, i)--(6, i)^^(i, 0)--(i, 6)); } [/asy] Find the number of ways to partition a $10\times10$ grid of cells into $5$ cell loops so that every cell of the grid belongs to exactly one cell loop.