264826domain: NAppears in sequencesa(n) = a(n-1) + 2*a(n-2) - a(n-3) - a(n-4) + 2*a(n-5) - 2*a(n-7) for n>7 and where a(0)=2, a(1)=3, a(2)=6, a(3)=10, a(4)=17, a(5)=29, a(6)=51.at n=21A285665