a(n) = 4^n - binomial(2*n,n).

Terms

a(0) =0a(1) =2a(2) =10a(3) =44a(4) =186a(5) =772a(6) =3172a(7) =12952a(8) =52666a(9) =213524a(10) =863820a(11) =3488872a(12) =14073060a(13) =56708264a(14) =228318856a(15) =918624304a(16) =3693886906a(17) =14846262964a(18) =59644341436a(19) =239532643144a(20) =961665098956