Numbers of the form (2i)! (2j)! / i! j! (i + j)!.

Terms

a(0) =1a(1) =2a(2) =4a(3) =6a(4) =10a(5) =12a(6) =20a(7) =28a(8) =40a(9) =70a(10) =72a(11) =84a(12) =90a(13) =140a(14) =198a(15) =220a(16) =252a(17) =264a(18) =308a(19) =504a(20) =572a(21) =728a(22) =858a(23) =924a(24) =1092a(25) =1560a(26) =1716a(27) =1820a(28) =1848a(29) =2520