a(n) = 9*4^n+1.

Terms

a(0) =10a(1) =37a(2) =145a(3) =577a(4) =2305a(5) =9217a(6) =36865a(7) =147457a(8) =589825a(9) =2359297a(10) =9437185a(11) =37748737a(12) =150994945a(13) =603979777a(14) =2415919105a(15) =9663676417a(16) =38654705665a(17) =154618822657a(18) =618475290625