INVERTi transform of central trinomial coefficients (A002426).

Terms

a(0) =0a(1) =1a(2) =2a(3) =2a(4) =4a(5) =8a(6) =18a(7) =42a(8) =102a(9) =254a(10) =646a(11) =1670a(12) =4376a(13) =11596a(14) =31022a(15) =83670a(16) =227268a(17) =621144a(18) =1706934a(19) =4713558a(20) =13072764a(21) =36398568a(22) =101704038a(23) =285095118a(24) =801526446a(25) =2259520830a(26) =6385455594a(27) =18086805002a(28) =51339636952a(29) =146015545604