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

1,2

Links

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Example

7^2=49 and 4+9=13 is prime.

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);

Mathematica

a={}; For[n=1, n<700, n++, If[PrimeQ[Plus@@IntegerDigits[7^n]], AppendTo[a, n]]]; a (* Vincenzo Librandi, Apr 17 2013 *)

Crossrefs

Cf. A076203, A134532, A134534, A134535.

Sequence in context: A121519 A098968 A032368 * A089953 A155133 A331669

Adjacent sequences: A134530 A134531 A134532 * A134534 A134535 A134536

Keyword

easy,nonn,base

Author

Paolo P. Lava and Giorgio Balzarotti, Oct 30 2007

Status

approved