A340741 - OEIS

%I #20 Feb 06 2021 22:09:56

%S 7,8,9,11,12,13,15,18,19,28,31,32,34,36,44,46,47,51,52,62,64,67,69,70,

%T 73,83,88,109,110,112,128,148,153,159,189,190,192,206,212,214,222,224,

%U 226,244,245,261,267,269,280,282,283,287,300,305,312,315,319,323,366,370,378,381,388,394,404

%N Numbers k such that A340740(k) is prime.

%H Robert Israel, <a href="/A340741/b340741.txt">Table of n, a(n) for n = 1..5000</a>

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

%p f:= proc(n) local k;

%p   add(`if`(igcd(k,n)=1, n mod k, 0),k=1..floor(n/2))

%p end proc:

%p select(t -> isprime(f(t)), [$1..1000]);

%t A340741[n_] :=

%t   Position[Table[

%t      PrimeQ[Sum[

%t        Mod[m, i]*Floor[1/GCD[i, m]], {i, Floor[(m - 1)/2]}]], {m, 1,

%t       n}], True] // Flatten;

%t A340741[404] (* _Robert P. P. McKone_, Jan 19 2021 *)

%o (PARI) isok(n) = isprime(sum(k=1, n\2, if (gcd(k, n)==1, n%k))); \\ _Michel Marcus_, Jan 18 2021

%Y Cf. A340731, A340740, A340742.

%K nonn

%O 1,1

%A _J. M. Bergot_ and _Robert Israel_, Jan 18 2021