A340741 Numbers k such that A340740(k) is prime.

7, 8, 9, 11, 12, 13, 15, 18, 19, 28, 31, 32, 34, 36, 44, 46, 47, 51, 52, 62, 64, 67, 69, 70, 73, 83, 88, 109, 110, 112, 128, 148, 153, 159, 189, 190, 192, 206, 212, 214, 222, 224, 226, 244, 245, 261, 267, 269, 280, 282, 283, 287, 300, 305, 312, 315, 319, 323, 366, 370, 378, 381, 388, 394, 404

Offset: 1

Author

J. M. Bergot and Robert Israel, Jan 18 2021

Keywords

• nonn

Examples

			a(3) = 9 is a term because A340740(9) = 2 is prime.
		

Links

• Robert Israel, Table of n, a(n) for n = 1..5000

Crossrefs

Cf. A340731, A340740, A340742.

Programs

Maple

f:= proc(n) local k;
  add(`if`(igcd(k,n)=1, n mod k, 0),k=1..floor(n/2))
end proc:
select(t -> isprime(f(t)), [$1..1000]);

Mathematica

A340741[n_] :=
  Position[Table[
     PrimeQ[Sum[
       Mod[m, i]*Floor[1/GCD[i, m]], {i, Floor[(m - 1)/2]}]], {m, 1,
      n}], True] // Flatten;
A340741[404] (* Robert P. P. McKone, Jan 19 2021 *)

PARI

isok(n) = isprime(sum(k=1, n\2, if (gcd(k, n)==1, n%k))); \\ Michel Marcus, Jan 18 2021