%I #15 Apr 24 2021 20:51:01
%S 4095,8191,12287,16383,20479,24575,28671,29183,29695,30207,30719,
%T 31231,31743,32255,32319,32383,32447,32511,32575,32639,32703,32711,
%U 32719,32727,32735,32743,32751,32759,32760,32761,32762,32763,32764,32765,32766,36863,40959,45055
%N Numbers having four 7's in base 8.
%H Robert Israel, <a href="/A043452/b043452.txt">Table of n, a(n) for n = 1..10000</a>
%p G:= proc(x,c,d) local L,m1,m2;
%p L:= convert(x,base,7);
%p m2:= nops(c);
%p m1:= min(nops(L),m2);
%p 8^d-1 + add((L[i]-7)*8^(c[i]-1),i=1..m1) - 7*add(8^(c[i]-1),i = m1+1..m2)
%p end proc:
%p sort([seq(seq(G(x,c,6), x = 0 .. 7^2-1), c=combinat:-choose(6,2))]); # _Robert Israel_, Sep 24 2017
%t Select[Range[0, 10^5], DigitCount[#, 8, 7]==4 &] (* _Vincenzo Librandi_, Sep 25 2017 *)
%o (PARI) isok(n) = #select(x->(x==7), digits(n, 8)) == 4; \\ _Michel Marcus_, Sep 25 2017
%Y Cf. A007094.
%K nonn,base
%O 1,1
%A _Clark Kimberling_
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