519588domain: NAppears in sequencesa(n) = 3*(n-2)*(n-3)*(3*n^2-3*n-8)/2.at n=18A064198Numbers k such that hexagonal numbers H(k) and H(k+1) sum to another hexagonal number.at n=4A251601a(n) = 2*a(n-1) + 2*a(n-3) + a(n-4) - 1, with a(0) = 0 = a(1), a(2) = 2, and a(3) = 3.at n=17A353582