Direct matrix (non-recursive) content of -n to n+1 symmetry matrices.

Terms

a(0) =0a(1) =0a(2) =2a(3) =0a(4) =6a(5) =6a(6) =0a(7) =12a(8) =12a(9) =156a(10) =0a(11) =20a(12) =20a(13) =420a(14) =820a(15) =0a(16) =30a(17) =30a(18) =930a(19) =1830a(20) =29730a(21) =0a(22) =42a(23) =42a(24) =1806a(25) =3570a(26) =79422a(27) =229362a(28) =0a(29) =56