a(n) is the nearest integer to sqrt(pi(10^n)) (see A006880).

Terms

a(0) =0a(1) =2a(2) =5a(3) =13a(4) =35a(5) =98a(6) =280a(7) =815a(8) =2400a(9) =7131a(10) =21332a(11) =64172a(12) =193928a(13) =588273a(14) =1790235a(15) =5463018a(16) =16710426a(17) =51220671a(18) =157289397a(19) =483795067a(20) =1490241458a(21) =4596440959a(22) =14193917243a(23) =43878472986a(24) =135777758736a(25) =420530985064