A156592 Product p*q of two primes with q = 2*p + 1.

10, 21, 55, 253, 1081, 1711, 3403, 5671, 13861, 15931, 25651, 34453, 60031, 64261, 73153, 108811, 114481, 126253, 158203, 171991, 258121, 351541, 371953, 392941, 482653, 518671, 703891, 822403, 853471, 869221, 933661, 1034641, 1104841

Offset: 1

Author

Reinhard Zumkeller, Feb 10 2009

Keywords

• nonn

Comments

Subsequence of A068443.

Products of Sophie Germain primes p with their corresponding safe primes 2p+1. The smallest prime factor of a(n) is (a(n) - phi(a(n)))/3 and the largest prime factor of a(n) is 2(a(n) - phi(a(n)))/3 + 1. - Wesley Ivan Hurt, Oct 03 2013

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..4000
• Math Marathon (MM95) (in Russian) [from Vladimir Joseph Stephan Orlovsky, May 12 2010]

Crossrefs

Cf. A005384, A005385. Subset of A001358.

Programs

Mathematica

lst={};Do[p=Prime[n];q=2*p+1;If[PrimeQ[q],AppendTo[lst,p*q]],{n,5!}];lst (* Vladimir Joseph Stephan Orlovsky, May 12 2010 *)

Formula

a(n) = A005384(n)*A005385(n).