In this note we propose a new algorithm for checking whether two counting functions on a free monoid $M_r$ of rank $r$ are equivalent modulo a bounded function. The previously known algorithm has time complexity $O(n)$ for all ranks $r>2$, however in case $r=2$ it was estimated only as $O(n^2)$. Here we apply a new approach, based on explicit basis expansion and weighted rectangles summation, which allows us to construct a much simpler algorithm with time complexity $O(n)$ for any $r\geq 2$.
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