548137914373domain: NAppears in sequencesExpansion of (1-x)^2/(1 - 4*x + 3*x^2 - x^3).at n=24A052544INVERT transform of [1, 0, 1, 3, 9, 27, 81, ...].at n=25A204200a(n) = Sum_{k=0..2*n} binomial(2*k,2*n-k).at n=18A391594