109143domain: NAppears in sequencesNumber of ways to build a contiguous building with n LEGO blocks of size 1 X 2 on top of a fixed block of the same size so that the building is symmetric after a rotation by 180 degrees.at n=10A123767G.f. A(x) satisfies: 1 = Sum_{n>=0} x^n * ((1+x)^n - A(x))^(n+1), where A(0) = 0.at n=9A307940