9990280domain: NAppears in sequencesNumbers X such that X^2 + Y^2 = 10^(2*k) + 1, with X > Y > 0 and k is the decimal digit length of X-1.at n=21A368418Number of 4-partitions of the fan graph F_{1,n} where every triangle uses exactly 2 parts.at n=16A390130