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(Product of twin primes - 1)/2.
5

%I #35 Feb 25 2018 20:11:08

%S 7,17,71,161,449,881,1799,2591,5201,5831,9521,11249,16199,18431,19601,

%T 25991,28799,36449,39761,48671,60551,88199,93311,106721,136241,162449,

%U 179999,190961,206081,217799,328049,337841,342791,368081,388961,520199,532511,551249,563921

%N (Product of twin primes - 1)/2.

%C This sequence is a subsequence of A102770.

%H Jason Kimberley, <a href="/A120876/b120876.txt">Table of n, a(n) for n = 1..5000</a>

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

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

%F a(n) = A102770(A107770(n)). - _Jason Kimberley_, Nov 10 2015

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

%o (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

%o (PARI) p=2; forprime(q=3, 1e3, if(q-p==2, print1(p*q\2", ")); p=q) \\ _Charles R Greathouse IV_, Apr 01 2016

%Y Cf. The subsequence A086870.

%K nonn

%O 1,1

%A _Lekraj Beedassy_, Jul 09 2006

%E Corrected by _T. D. Noe_, Oct 25 2006

%E Edited by _Jason Kimberley_, Oct 23 2015