91280domain: NAppears in sequencesNumber of ways to place 2 nonattacking bishops on an n X n board.at n=20A172123a(n) = min {i - j} where i*j is a factorization of the n-th partial product of the semiprimes (A112141).at n=14A317490G.f. satisfies A(x) = 1 + 2*x*A(x) + x^3*A(x)^3.at n=11A367112