A230606 - OEIS

A230606

Numbers n such that sigma(n) = k*(n+1) for some integer k.

2, 3, 5, 7, 11, 13, 17, 19, 20, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 104, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263

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OFFSET

1,1

COMMENTS

Numbers n such that A108775(n) = floor(sigma(n) / n) = sigma(n) mod n = A054024(n).

Union primes (A000040) and composite numbers A045768 (k = 1 for primes p, k = 2 for composite numbers).

LINKS

Jaroslav Krizek, Table of n, a(n) for n = 1..1000

EXAMPLE

20 is in sequence because sigma(20) = 42 = 2*21.

MATHEMATICA

Select[Range[300], Divisible[DivisorSigma[1, #], #+1]&] (* Harvey P. Dale, May 28 2019 *)