19306982domain: NAppears in sequencesa(0)=0, a(1)=2 then a(n) = a(n-2) + 2*sqrt(8*a(n-1)^2 + 8*a(n-1) + 1).at n=10A053141a(n) = (2*Pell(n+1) - (1+(-1)^n))/4.at n=20A105635Numbers n such that n^2 + (n+1)^2 is equal to the sum of the hexagonal numbers H(m) and H(m+1) for some m.at n=5A251867