30409domain: NAppears in sequencesa(n) = floor(C(4n,2n)/C(4n,n)).at n=20A024501Composite and every divisor (except 1) contains the digit 4.at n=21A062670Expansion of (1-x+x^2)/((1+x^2)(1-4x+x^2)).at n=8A099489Numbers n such that n!!! - 3^9 is prime, where n!3 = n!!! is a triple factorial number (A007661).at n=28A267029