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%I #23 Jun 13 2019 03:34:02
%S 1,3,6,10,15,22,30,39,49,60,73,87,102,118,135,154,174,195,217,240,265,
%T 291,318,346,376,407,439,472,506,542,579,617,656,696,738,781,825,870,
%U 916,964,1013,1063,1114,1166,1220,1275,1331,1388,1447,1507,1568,1630,1693
%N Index of 7^n within sequence of numbers of form 5^i*7^j.
%H Robert Israel, <a href="/A025723/b025723.txt">Table of n, a(n) for n = 0..10000</a>
%F a(n) = 1 + Sum_{k=1..n} ceiling(k*log_5(7)). - _Robert Israel_, Nov 16 2016
%p ListTools:-PartialSums([1,seq(ceil(k*log[5](7)),k=1..100)]); # _Robert Israel_, Nov 16 2016
%t Table[1 + Sum[Ceiling[k Log[5, 7]], {k, n}], {n, 0, 52}] (* _Michael De Vlieger_, Nov 16 2016 *)
%o (PARI) a(n)=my(N=1); n+1+sum(i=1, n, logint(N*=7, 5)); \\ _Charles R Greathouse IV_, Jan 11 2018
%o (PARI) first(n)=my(s, N=1/7); vector(n+1, i, s+=logint(N*=7, 5)+1) \\ _Charles R Greathouse IV_, Jan 11 2018
%Y Cf. A025708, A022330, A022331, etc.
%K nonn,easy
%O 0,2
%A _David W. Wilson_
%E Offset changed to 0 by _Robert Israel_, Nov 16 2016
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