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

Terms

a(0) =1a(1) =2a(2) =5a(3) =16a(4) =60a(5) =254a(6) =1188a(7) =6043a(8) =33080a(9) =193249a(10) =1197001a(11) =7819995a(12) =53648847a(13) =385090323a(14) =2883045424a(15) =22451716833a(16) =181437812058