A067730 Numbers n such that sigma(n-1) + sigma(n+1) = sigma(2n).

309, 425, 2135, 2913, 6861, 20155, 37415, 45155, 74875, 329841, 4720281, 6385749, 7030911, 11606649, 13954745, 20920075, 22436225, 22937785, 37760631, 38748291, 81607505, 85815925, 95375589, 114195965, 115314295, 122491401, 132765639

Offset

1,1

Comments

Conjecture: sequence contains odd values only. - Benoit Cloitre, Feb 18 2002

Links

Donovan Johnson, Table of n, a(n) for n = 1..100

Example

sigma(309-1) + sigma(309+1) = 672+576 = sigma(2*309), so 309 is a term of the sequence.

Mathematica

Select[Range[10^6], DivisorSigma[1, # - 1] + DivisorSigma[1, # + 1] == DivisorSigma[1, 2# ] &]

Prog

(PARI) a067730(m) = for(n=2, m, if(sigma(n-1)+sigma(n+1)==sigma(2*n), print1(n, ", "))) a067730(10^7)

Crossrefs

Sequence in context: A060980 A061881 A121322 * A183627 A184544 A251345

Adjacent sequences: A067727 A067728 A067729 * A067731 A067732 A067733

Keyword

nonn

Author

Joseph L. Pe, Feb 05 2002

Extensions

More terms from Klaus Brockhaus, Feb 07 2002

Edited and extended by Robert G. Wilson v, Feb 08 2002

a(21)-a(27) from Donovan Johnson, Jan 31 2009

Status

approved