A326391 Lesser of twin primes p >= 3 for which sigma(p+1)/sigma(p-1) reaches record value, where sigma(n) is the divisor sum function (A000203).

3, 7559, 42839, 55439, 110879, 415799, 1713599, 1940399, 2489759, 6652799, 6846839, 15855839, 31600799, 85765679, 232792559, 845404559, 1470268799, 6299092799, 10708457759, 17459441999, 32125373279, 135019684799, 439977938399, 449755225919, 1799020903679, 2126560035599, 2835413380799, 6278415343199

Offset: 1

Author

Amiram Eldar, Sep 11 2019

Keywords

• nonn

Comments

Garcia et al. proved that assuming Dickson's conjecture, {sigma(p+1)/sigma(p-1) : p and p+2 are prime} is dense in [0, oo), and thus this sequence is infinite.

Examples

			The values of sigma(p+1)/sigma(p-1) for the first terms are 2.333... < 2.539... < 2.621... < 2.734... < 2.836...
		

Links

• Stephan Ramon Garcia, Florian Luca, Kye Shi, Gabe Udell, Primitive root bias for twin primes II: Schinzel-type theorems for totient quotients and the sum-of-divisors function, arXiv:1906.05927 [math.NT], 2019.
• Wikipedia, Dickson's conjecture.

Crossrefs

Cf. A000203, A001359, A006512, A008332, A008333, A326356.

Programs

Mathematica

s = {}; rm = 0; p = 2; Do[q = NextPrime[p]; If[q - p != 2, p = q; Continue[]]; r = DivisorSigma[1, p + 1]/DivisorSigma[1, p - 1]; If[r > rm, rm = r; AppendTo[s, p]]; p = q, {10^6}]; s

Extensions

a(22)-a(28) from Giovanni Resta, Nov 01 2019