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