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%I #8 Apr 17 2013 16:21:17
%S 1,2,4,8,10,12,18,19,22,24,25,32,33,35,45,56,57,58,59,60,72,73,76,81,
%T 82,84,88,100,102,103,104,105,117,118,125,136,138,142,147,149,162,188,
%U 190,192,195,201,203,206,210,212,232,240,246,252,262,264,265,269,270,280
%N Numbers n such that the sum of the digits of 7^n is prime.
%H Vincenzo Librandi, <a href="/A134533/b134533.txt">Table of n, a(n) for n = 1..1000</a>
%e 7^2=49 and 4+9=13 is prime.
%p P:=proc(n) local cont,i,k,w; if isprime(n) then cont:=0; while cont<1000 do cont:=cont+1; w:=0; k:=n^cont; while k>0 do w:=w+k-(trunc(k/10)*10); k:=trunc(k/10); od; if isprime(w) then print(cont); fi; od; fi; end: P(7);
%t a={}; For[n=1, n<700, n++, If[PrimeQ[Plus@@IntegerDigits[7^n]], AppendTo[a, n]]];a (* _Vincenzo Librandi_, Apr 17 2013 *)
%Y Cf. A076203, A134532, A134534, A134535.
%K easy,nonn,base
%O 1,2
%A _Paolo P. Lava_ and _Giorgio Balzarotti_, Oct 30 2007
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