50466domain: NAppears in sequencesNumber of ways to place 3 nonattacking bishops on a 3 X n board.at n=23A172207Let b(n) be the number of permutations {c_1..c_n} of {1..n} for which c_1 - c_2 + ... + (-1)^(n-1)*c_n are pentagonal numbers (A000326). Then a(n) = b(n)/A010551(n).at n=21A294812