cp's OEIS Frontend

A381507 Squarefree numbers k such that the sum of 1/(p-1) over the prime divisors p of k is 1.

%I A381507 #25 Apr 27 2025 09:50:01
%S A381507 2,1365,73815,6702045,8788065,26241285,32426205,237539445,269409855,
%T A381507 445317015,475231515,709296105,1085962395,1329722835,1447857915,
%U A381507 2403281595,3255993615,5145721455,5254163355,5824953435,6560751435,7176232455,7703697855,8332635255,8542035645
%N A381507 Squarefree numbers k such that the sum of 1/(p-1) over the prime divisors p of k is 1.
%C A381507 Squarefree terms of A380888.
%C A381507 All terms > 2 are odd.
%e A381507 1365 is a term because 1365 = 3 * 5 * 7 * 13 and 1/(3-1) + 1/(5-1) + 1/(7-1) + 1/(13-1) = 1/2 + 1/4 + 1/6 + 1/12 = 1.
%p A381507 filter:= proc(n) local F,t;
%p A381507    F:=ifactors(n)[2];
%p A381507    if F[..,2] <> [1$nops(F)] then return false fi;
%p A381507    add(1/(t-1),t=F[..,1]) = 1
%p A381507 end proc:
%p A381507 select(filter, [2, seq(i,i=1..10^8,2)]);
%Y A381507 Intersection of A005117 and A380888.
%K A381507 nonn
%O A381507 1,1
%A A381507 _Robert Israel_, Apr 23 2025
%E A381507 More terms from _Giorgos Kalogeropoulos_, Apr 27 2025