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

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =16a(5) =57a(6) =231a(7) =1023a(8) =4926a(9) =25483a(10) =140601a(11) =822422a(12) =5074015a(13) =32881868a(14) =223027542a(15) =1578435549a(16) =11625317128a(17) =88894615929a(18) =704269188135