a(n) = Sum_{k=1..n} (binomial(n, k) * 3^k) (mod 4^k).

Terms

a(0) =3a(1) =11a(2) =39a(3) =131a(4) =419a(5) =1391a(6) =4283a(7) =12087a(8) =96999a(9) =291795a(10) =897215a(11) =3595915a(12) =10086699a(13) =43994727a(14) =129293667a(15) =344359327a(16) =1809740431a(17) =6153300075a(18) =17431702983a(19) =44227307811a(20) =383495816995