a(n) = (4^(n+1) + (-1)^n + 5)/10.

Terms

a(0) =1a(1) =2a(2) =7a(3) =26a(4) =103a(5) =410a(6) =1639a(7) =6554a(8) =26215a(9) =104858a(10) =419431a(11) =1677722a(12) =6710887a(13) =26843546a(14) =107374183a(15) =429496730a(16) =1717986919a(17) =6871947674a(18) =27487790695a(19) =109951162778a(20) =439804651111