a(n) = ceiling(10*log_10(n)).

Terms

a(0) =0a(1) =4a(2) =5a(3) =7a(4) =7a(5) =8a(6) =9a(7) =10a(8) =10a(9) =10a(10) =11a(11) =11a(12) =12a(13) =12a(14) =12a(15) =13a(16) =13a(17) =13a(18) =13a(19) =14a(20) =14a(21) =14a(22) =14a(23) =14a(24) =14a(25) =15a(26) =15a(27) =15a(28) =15a(29) =15