This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A239009 #38 Jan 21 2020 00:29:33
%S A239009 0,2,4,7,9,12,14,16,17,23,34,36,38,43,77,88,216,350,979,24186,28678,
%T A239009 134759,205829,374627,2200364,16625243,29451854,162613199,8078176309,
%U A239009 9252290259,17556077280,49718535383,51616746477,54585993918
%N A239009 Exponents m such that the decimal expansion of 4^m exhibits its first zero from the right later than any previous exponent.
%C A239009 Assume that a zero precedes all decimal expansions. This will take care of those cases in A030701.
%C A239009 Inspired by the seqfan list discussion Re: "possible sequence", beginning with David Wilson 7:57 PM Mar 06 2014 and continued by _M. F. Hasler_, _Allan C. Wechsler_ and _Franklin T. Adams-Watters_.
%C A239009 Not just twice A031142, although {16625243, 29451854, 162613199, 9252290259, 51616746477, 54585993918, 146235898847, 1360645542292} are possible candidates.
%C A239009 Location of first zeros (from the right) of terms: 2, 3, 4, 6, 7, 9, 10, 11, 12, 13, 14, 23, 24, 27, 30, 39, 53, 58, 94, 113, 120, 121, 122, 139, 165, 177, 192, 213, 217, 228, 229, 230, 250, 251. - _Chai Wah Wu_, Jan 08 2020
%t A239009 f[n_] := Position[ Reverse@ Join[{0}, IntegerDigits[ PowerMod[4, n, 10^500]]], 0, 1, 1][[1, 1]]; k = mx = 0; lst = {}; While[k < 100000001, c = f[k]; If[c > mx, mx = c; AppendTo[ lst, k]; Print@ k]; k++]; lst
%Y A239009 Cf. A000302, A030701, A020665, A031142, A239008, A239010, A239011, A239012, A239013, A239014, A239015.
%K A239009 nonn,base,more
%O A239009 1,2
%A A239009 _Charles R Greathouse IV_ and _Robert G. Wilson v_, Mar 14 2014
%E A239009 a(28)-a(30) from _Bert Dobbelaere_, Jan 21 2019
%E A239009 a(31)-a(34) from _Chai Wah Wu_, Jan 08 2020