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 A077612 Number of adjacent pairs of form (even,even) among all permutations of {1,2,...,n}. 3
 0, 0, 0, 12, 48, 720, 4320, 60480, 483840, 7257600, 72576000, 1197504000, 14370048000, 261534873600, 3661488230400, 73229764608000, 1171676233728000, 25609494822912000, 460970906812416000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 LINKS FORMULA a(n) = floor(n/2)*floor(n/2-1)*(n-1)!. Proof: There are floor(n/2)*floor(n/2-1) pairs (r, s) with r and s even and distinct. For each pair, there are n-1 places it can occur in a permutation and (n-2)! possible arrangements of the other numbers. CROSSREFS Cf. A077611, A077613. Sequence in context: A222359 A002899 A222966 * A041272 A022282 A009959 Adjacent sequences:  A077609 A077610 A077611 * A077613 A077614 A077615 KEYWORD nonn AUTHOR Leroy Quet Frank Ruskey and Dean Hickerson (dean.hickerson(AT)yahoo.com), Nov 11 2002 STATUS approved

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