a(n) = A131668(n) - (2*n+1).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =18a(6) =36a(7) =54a(8) =72a(9) =180a(10) =378a(11) =576a(12) =774a(13) =972a(14) =2970a(15) =4968a(16) =6966a(17) =8964a(18) =19962a(19) =39960a(20) =59958a(21) =79956a(22) =99954a(23) =299952a(24) =499950a(25) =699948a(26) =899946a(27) =1999944a(28) =3999942a(29) =5999940