G.f.: Sum_{n>=0} x^n / (1 - n*x - n^2*x^2).

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =15a(5) =52a(6) =207a(7) =923a(8) =4532a(9) =24271a(10) =140581a(11) =874434a(12) =5806557a(13) =40955973a(14) =305544958a(15) =2402139329a(16) =19837601155a(17) =171598571288