A134532 Numbers k such that the sum of the digits of 5^k is prime.

1, 2, 4, 5, 6, 7, 13, 19, 20, 22, 26, 27, 29, 33, 34, 36, 41, 43, 44, 50, 54, 55, 58, 59, 60, 66, 69, 70, 74, 75, 81, 85, 91, 95, 97, 99, 100, 101, 110, 112, 125, 127, 129, 131, 133, 134, 136, 142, 143, 145, 146, 148, 153, 156, 157, 163, 165, 178, 187, 189, 190, 196

Offset: 1

Author

Paolo P. Lava and Giorgio Balzarotti, Oct 30 2007

Keywords

• easy
• nonn
• base

Examples

			5^2=25 and 2+5=7 is prime.
		

Links

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

Crossrefs

Cf. A076203, A134533, A134534, A134535.

Programs

Maple

P:=proc(k) if isprime(convert(convert(5^k,base,10),`+`)) then k; fi; end: seq(P(n),n=1..200);

Mathematica

a={};For[n=1, n<200, n++, If[PrimeQ[Plus@@IntegerDigits[5^n]], AppendTo[a, n]]];a (* Vincenzo Librandi, Apr 17 2013 *)

Extensions

Corrected name and new Maple code by_Paolo P. Lava_, Jun 24 2024