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 A025742 #21 Jan 10 2018 16:13:21 %S A025742 1,3,7,12,19,28,38,50,64,79,96,115,135,157,181,206,233,262,292,324, %T A025742 358,393,430,469,509,551,595,640,687,736,786,838,892,947,1004,1063, %U A025742 1123,1185,1249,1314,1381,1450,1520,1592,1666,1741,1818,1897,1977,2059,2143,2228 %N A025742 a(n) is the index of 10^n within sequence of numbers of form 4^i*10^j. %C A025742 Indices m for which A025649(m) is 0. - _Michel Marcus_, Jan 10 2018 %H A025742 Charles R Greathouse IV, <a href="/A025742/b025742.txt">Table of n, a(n) for n = 0..10000</a> %e A025742 The index of 10^0 in A025621 is 1, so a(0)=1. %e A025742 The index of 10^1 in A025621 is 3, so a(1)=3. %e A025742 The index of 10^2 in A025621 is 7, so a(2)=7. %o A025742 (PARI) a(n)=my(N=1); n+1+sum(i=1,n, logint(N*=10,4)); \\ _Charles R Greathouse IV_, Jan 10 2018 %o A025742 (PARI) first(n)=my(s,N=1/10); vector(n+1,i, s+=logint(N*=10,4)+1) \\ _Charles R Greathouse IV_, Jan 10 2018 %Y A025742 Cf. A025621, A025649, A025686. %K A025742 nonn,easy %O A025742 0,2 %A A025742 _David W. Wilson_ %E A025742 Offset changed by _Michel Marcus_, Jan 10 2018