a(n) = floor(log_10(2^(2^n))).

Terms

a(0) =0a(1) =1a(2) =2a(3) =4a(4) =9a(5) =19a(6) =38a(7) =77a(8) =154a(9) =308a(10) =616a(11) =1233a(12) =2466a(13) =4932a(14) =9864a(15) =19728a(16) =39456a(17) =78913a(18) =157826a(19) =315652a(20) =631305a(21) =1262611a(22) =2525222a(23) =5050445a(24) =10100890a(25) =20201781a(26) =40403562a(27) =80807124a(28) =161614248a(29) =323228496