A134533 Numbers n such that the sum of the digits of 7^n is prime.
1, 2, 4, 8, 10, 12, 18, 19, 22, 24, 25, 32, 33, 35, 45, 56, 57, 58, 59, 60, 72, 73, 76, 81, 82, 84, 88, 100, 102, 103, 104, 105, 117, 118, 125, 136, 138, 142, 147, 149, 162, 188, 190, 192, 195, 201, 203, 206, 210, 212, 232, 240, 246, 252, 262, 264, 265, 269, 270, 280
Offset: 1
Examples
7^2=49 and 4+9=13 is prime.
Links
- Vincenzo Librandi, Table of n, a(n) for n = 1..1000
Programs
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Maple
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);
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Mathematica
a={}; For[n=1, n<700, n++, If[PrimeQ[Plus@@IntegerDigits[7^n]], AppendTo[a, n]]];a (* Vincenzo Librandi, Apr 17 2013 *)