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A292931 Numbers n such that the sum of digits of 3^n (A004166) is divisible by 7.

%I A292931 #23 Mar 01 2018 18:36:10
%S A292931 25,26,30,32,47,58,79,81,87,89,102,123,141,144,145,151,164,176,178,
%T A292931 193,201,227,239,242,257,264,282,289,300,306,319,324,329,335,336,338,
%U A292931 348,351,358,365,395,403,437,441,450,460,468,484,489,492,495,517,518,541,542,544,554,555,563,565,570
%N A292931 Numbers n such that the sum of digits of 3^n (A004166) is divisible by 7.
%H A292931 Chai Wah Wu, <a href="/A292931/b292931.txt">Table of n, a(n) for n = 1..10000</a>
%H A292931 Mathematics StackExchange, <a href="https://math.stackexchange.com/questions/2433244">Is sum of digits of 3^1000 divisible by 7?</a>
%e A292931 a(3) = 30 is in the sequence because 3^30 = 205891132094649 has sum of digits 63, which is divisible by 7.
%p A292931 select(n -> convert(convert(3^n,base,10),`+`) mod 7 = 0, [$1..1000]);
%t A292931 Select[Range[600],Divisible[Total[IntegerDigits[3^#]],7]&] (* _Harvey P. Dale_, Mar 01 2018 *)
%o A292931 (PARI) isok(n) = !(sumdigits(3^n) % 7); \\ _Michel Marcus_, Sep 27 2017
%o A292931 (Python)
%o A292931 from __future__ import division
%o A292931 A292931_list = [n for n in range(1000) if not sum(int(d) for d in str(3**n)) % 7] # _Chai Wah Wu_, Sep 28 2017
%Y A292931 Cf. A004166.
%K A292931 nonn,base
%O A292931 1,1
%A A292931 _Robert Israel_, Sep 27 2017