27643067007domain: NAppears in sequencesE.g.f.: Sum_{n>=0} d^n/dx^n (x + x^2)^(2*n) / (2*n)!.at n=12A215125a(n) = Sum_{k=0..floor(n/3)} (n-k)!/(k! * (n-3*k)!).at n=24A358560