Second elementary symmetric function of the first n terms of (1,1,2,2,3,3,4,4,...).

Terms

a(0) =1a(1) =5a(2) =13a(3) =31a(4) =58a(5) =106a(6) =170a(7) =270a(8) =395a(9) =575a(10) =791a(11) =1085a(12) =1428a(13) =1876a(14) =2388a(15) =3036a(16) =3765a(17) =4665a(18) =5665a(19) =6875a(20) =8206a(21) =9790a(22) =11518a(23) =13546a(24) =15743a(25) =18291a(26) =21035a(27) =24185a(28) =27560a(29) =31400