Binomial(2n, n) - 2 mod n^4.

Terms

a(0) =0a(1) =4a(2) =18a(3) =68a(4) =250a(5) =922a(6) =1029a(7) =580a(8) =2691a(9) =4754a(10) =2662a(11) =8474a(12) =4394a(13) =10294a(14) =2518a(15) =49732a(16) =29478a(17) =65074a(18) =123462a(19) =128818a(20) =6535a(21) =93174a(22) =36501a(23) =12058a(24) =187750a(25) =162582a(26) =297936a(27) =273782a(28) =536558a(29) =741422