a(n) = (Sum_{i=0..n} 5^i) + 1 - (Sum_{i=0..n} 5^i) mod 2.

Terms

a(0) =1a(1) =7a(2) =31a(3) =157a(4) =781a(5) =3907a(6) =19531a(7) =97657a(8) =488281a(9) =2441407a(10) =12207031a(11) =61035157a(12) =305175781a(13) =1525878907a(14) =7629394531a(15) =38146972657a(16) =190734863281a(17) =953674316407