A234641 Odd numbers k such that sigma(sigma(k^2)) is odd.

1, 9, 38423, 104855, 247863, 313929, 345807, 376095, 469623, 623615, 787127, 943695, 985369, 1606281, 1754039, 1933815, 2034423, 2181409, 3043401, 5147241, 5545617, 5612535, 6385703, 7084143, 8868321, 10606679, 11470511, 11954409, 12276745, 12794655, 13213921, 14142695, 15512065, 15737953, 15786351, 16844135

Offset

1,2

Comments

The sum of divisors of a square is always odd, therefore these numbers have the property that x = n^2, y = sigma(x) and z = sigma(y) are all three odd.

This is the subsequence of odd terms of A008847.

Links

Amiram Eldar, Table of n, a(n) for n = 1..316 (terms 1..122 from M. F. Hasler)

Formula

a(n) = sqrt(A231484(n)). - Amiram Eldar, Mar 15 2026

Crossrefs

Cf. A000203, A000290, A008847, A008848, A163763, A231484.

Sequence in context: A053931 A053921 A297563 * A242853 A198478 A159398

Adjacent sequences: A234638 A234639 A234640 * A234642 A234643 A234644

Keyword

nonn

Author

M. F. Hasler, Dec 28 2013

Status

approved