A permutation pattern that illustrates the strong law of small numbers
Abstract
We obtain an explicit formula for the number of permutations of [n] that avoid the barred pattern bar143bar52. A curious feature of its counting sequence, 1, 1, 2, 5, 14, 43, 145, 538, 2194,..., is that the displayed terms agree with A122993 in the On-Line Encyclopedia of Integer Sequences, but the two sequences diverge thereafter.
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