C(2n-1,n-1) mod n^4.

Terms

a(0) =0a(1) =3a(2) =10a(3) =35a(4) =126a(5) =462a(6) =1716a(7) =2339a(8) =4627a(9) =2378a(10) =1332a(11) =4238a(12) =2198a(13) =5148a(14) =1260a(15) =57635a(16) =14740a(17) =85026a(18) =61732a(19) =64410a(20) =100509a(21) =163716a(22) =158172a(23) =171918a(24) =93876a(25) =309780a(26) =148969a(27) =444220a(28) =268280a(29) =370712