A280587 Composite numbers k such that phi(x) = phi(k) has only 2 solutions: x = k and x = 2*k.

81, 121, 343, 361, 529, 649, 841, 961, 1219, 1331, 1537, 1633, 1837, 1849, 1909, 1969, 2047, 2209, 2401, 2497, 2773, 2809, 2959, 3113, 3127, 3151, 3223, 3421, 3481, 3487, 3841, 3901, 3953, 4189, 4321, 4399, 4531, 4661, 4741, 4829, 4897, 4913, 5041, 5129, 5137, 5191, 5269, 5401, 5539, 5753

Offset: 1

Author

Thomas Ordowski and Altug Alkan, Jan 06 2017

Keywords

• nonn

Comments

All terms are odd.

If q > 7 is in A005385, then q^2 is in the sequence.

The sequence has a positive natural density. - Information from Carl Pomerance, Jan 07 2017

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000010, A005385, A058340 (such primes).