A088724 Numbers having exactly one divisor d>1 such that also d+1 is a divisor.

6, 18, 20, 40, 54, 56, 66, 78, 80, 100, 102, 110, 112, 114, 138, 140, 160, 162, 174, 182, 186, 198, 200, 222, 224, 234, 246, 258, 260, 272, 282, 318, 320, 340, 354, 364, 366, 392, 400, 402, 414, 426, 438, 448, 460, 474, 486, 498, 500, 506, 520, 522, 534, 544

Offset: 1

Author

Reinhard Zumkeller, Oct 12 2003

Keywords

• nonn

Comments

Subsequence of A088723.

The numbers of terms not exceeding 10^k, for k = 1, 2, ..., are 1, 10, 100, 976, 9712, 97140, 971139, 9711054, 97109111, 971091745, ... . Apparently, the asymptotic density of this sequence exists and equals 0.097109... . - Amiram Eldar, Jul 09 2022

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A088722, A088723, A088725, A088726.

Programs

Mathematica

Select[Range[600],Count[Differences[Rest[Divisors[#]]],1]==1&] (* Harvey P. Dale, Sep 05 2015 *)

Formula

A088722(a(n)) = 1.

Extensions

Extended by Ray Chandler, May 29 2008