1335857546domain: NAppears in sequencesa(1)=1, a(2)=2, a(3)=11, a(4)=19; a(n) = a(n-4) + sqrt(60*a(n-2)^2 + 60*a(n-2) + 1) for n >= 5.at n=20A103200Numbers k such that 60*k^2 + 60*k + 1 is a square.at n=21A105076