Number of ways of writing 0 as Sum_{k=-n..n} e(k)*k, where e(k) is 0 or 1.

Terms

a(0) =2a(1) =4a(2) =8a(3) =20a(4) =52a(5) =152a(6) =472a(7) =1520a(8) =5044a(9) =17112a(10) =59008a(11) =206260a(12) =729096a(13) =2601640a(14) =9358944a(15) =33904324a(16) =123580884a(17) =452902072a(18) =1667837680a(19) =6168510256a(20) =22903260088a(21) =85338450344a(22) =318995297200