A New Quantity Counted by OEIS Sequence A006012
Abstract
We prove an existing conjecture that the sequence defined recursively by a1=1, a2=2, an=4an-1-2an-2 counts the number of length-n permutations avoiding the four generalized permutation patterns 1-32-4, 1-42-3, 2-31-4, and 2-41-3.
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