a(n) = binomial(2*n+2,n) - 2*binomial(2*n,n) + binomial(2*n-2,n).

Terms

a(0) =0a(1) =0a(2) =4a(3) =20a(4) =85a(5) =344a(6) =1365a(7) =5368a(8) =21021a(9) =82160a(10) =320892a(11) =1253240a(12) =4896034a(13) =19137104a(14) =74847175a(15) =292929840a(16) =1147223325a(17) =4496006880a(18) =17631691440a(19) =69189377400a(20) =271676550390