Second elementary symmetric function of the first n terms of (1,2,2,3,3,4,4,5,5...).

Terms

a(0) =2a(1) =8a(2) =23a(3) =47a(4) =91a(5) =151a(6) =246a(7) =366a(8) =540a(9) =750a(10) =1037a(11) =1373a(12) =1813a(13) =2317a(14) =2956a(15) =3676a(16) =4566a(17) =5556a(18) =6755a(19) =8075a(20) =9647a(21) =11363a(22) =13378a(23) =15562a(24) =18096a(25) =20826a(26) =23961a(27) =27321a(28) =31145a(29) =35225