A120876 (Product of twin primes - 1)/2.

7, 17, 71, 161, 449, 881, 1799, 2591, 5201, 5831, 9521, 11249, 16199, 18431, 19601, 25991, 28799, 36449, 39761, 48671, 60551, 88199, 93311, 106721, 136241, 162449, 179999, 190961, 206081, 217799, 328049, 337841, 342791, 368081, 388961, 520199, 532511, 551249, 563921

Offset: 1

Author

Lekraj Beedassy, Jul 09 2006

Keywords

• nonn

Comments

This sequence is a subsequence of A102770.

Links

• Jason Kimberley, Table of n, a(n) for n = 1..5000

Crossrefs

Cf. The subsequence A086870.

Programs

Mathematica

(Times@@#-1)/2&/@Select[Partition[Prime[Range[200]], 2,1],Last[#]- First[#]== 2&] (* Harvey P. Dale, Jun 26 2011 *)

PARI

for(n=1, 200, if(prime(n+1)-prime(n)==2, print1((prime(n)*prime(n+1)-1)/2", "))) \\ Altug Alkan, Oct 23 2015

PARI

p=2; forprime(q=3, 1e3, if(q-p==2, print1(p*q\2", ")); p=q) \\ Charles R Greathouse IV, Apr 01 2016

Formula

a(n) = A120875(n)/2 = A075369(n)/2-1 = A075369(n)^2/2-1.

a(n) = 18*A002822(n-1)^2-1, for n>1.

a(n) = A102770(A107770(n)). - Jason Kimberley, Nov 10 2015

Extensions

Corrected by T. D. Noe, Oct 25 2006

Edited by Jason Kimberley, Oct 23 2015