47770domain: NAppears in sequencesNumber of strictly increasing arrangements of 5 nonzero numbers in -(n+3)..(n+3) with sum zero.at n=26A188124a(1) = 1; a(n > 1) = Sum_{0 < k < n} (-1)^(n - k - 1) a(n - k) Sum_{d|k} a(d).at n=30A317853Numbers k such that sigma_2(k^2 + 1) == 0 (mod k).at n=39A360105