Even Catalan numbers divided by 2.

Terms

a(0) =1a(1) =7a(2) =21a(3) =66a(4) =715a(5) =2431a(6) =8398a(7) =29393a(8) =104006a(9) =371450a(10) =1337220a(11) =17678835a(12) =64822395a(13) =238819350a(14) =883631595a(15) =3282060210a(16) =12233133510a(17) =45741281820a(18) =171529806825a(19) =644952073662