a(n) = 2*5^n+1.

Terms

a(0) =3a(1) =11a(2) =51a(3) =251a(4) =1251a(5) =6251a(6) =31251a(7) =156251a(8) =781251a(9) =3906251a(10) =19531251a(11) =97656251a(12) =488281251a(13) =2441406251a(14) =12207031251a(15) =61035156251a(16) =305175781251