a(n) = T(n,n) + T(n,n+1) + ... + T(n,2n), T given by A027960.

Terms

a(0) =1a(1) =4a(2) =9a(3) =21a(4) =46a(5) =99a(6) =209a(7) =436a(8) =901a(9) =1849a(10) =3774a(11) =7671a(12) =15541a(13) =31404a(14) =63329a(15) =127501a(16) =256366a(17) =514939a(18) =1033449a(19) =2072676a(20) =4154701a(21) =8324529a(22) =16673534a(23) =33386671a(24) =66837421a(25) =133778524a(26) =267724809a(27) =535721061a(28) =1071881326a(29) =2144473299