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

2, 1365, 73815, 6702045, 8788065, 26241285, 32426205, 237539445, 269409855, 445317015, 475231515, 709296105, 1085962395, 1329722835, 1447857915, 2403281595, 3255993615, 5145721455, 5254163355, 5824953435, 6560751435, 7176232455, 7703697855, 8332635255, 8542035645

Offset

1,1

Comments

Squarefree terms of A380888.

All terms > 2 are odd.

Links

Table of n, a(n) for n=1..25.

Example

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.

Maple

filter:= proc(n) local F, t;
   F:=ifactors(n)[2];
   if F[.., 2] <> [1$nops(F)] then return false fi;
   add(1/(t-1), t=F[.., 1]) = 1
end proc:
select(filter, [2, seq(i, i=1..10^8, 2)]);

Crossrefs

Intersection of A005117 and A380888.

Sequence in context: A202866 A265587 A244614 * A094477 A350806 A135129

Adjacent sequences: A381504 A381505 A381506 * A381508 A381509 A381510

Keyword

nonn

Author

Robert Israel, Apr 23 2025

Extensions

More terms from Giorgos Kalogeropoulos, Apr 27 2025

Status

approved