This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A086870 #19 Nov 07 2015 03:22:51 %S A086870 7,17,71,449,881,2591,9521,39761,106721,179999,206081,342791,388961, %T A086870 596231,847601,1292831,2268449,2571911,2836961,3612671,6223391, %U A086870 6329681,6415361,8520191,8946449,9409121,10342151,12550049,16485281,18800711 %N A086870 Primes equal to a product of twin primes minus 1 divided by 2. %C A086870 From _Jason Kimberley_, Oct 22 2015 (Start) %C A086870 Prime elements of A120876. %C A086870 For each p in this list, A001221(2p) = A001222(2p) = A001221(2p+1) = A001222(2p+1) = 2. %C A086870 2*a(n) is a subsequence of A103533. They first differ when 313619 is not in this sequence, but 2*313619 = 627238 = A103533(12). %C A086870 (End) %H A086870 Jason Kimberley, <a href="/A086870/b086870.txt">Table of n, a(n) for n = 1..10000</a> %F A086870 Primes of the form (t1*t2-1)/2, where t1, t2 are twin primes. %e A086870 t1 = 71,t2 = 73, (71*73-1)/2 = 5182/2 = 2591 = prime. %t A086870 Select[(Times[#, # + 2] - 1)/2 &@ Select[Prime@ Range@ 1000, PrimeQ[# + 2] &], PrimeQ] (* _Michael De Vlieger_, Nov 06 2015 *) %o A086870 (PARI) for(n=1, 1e3, if(prime(n+1)-prime(n)==2 && isprime(k=(prime(n)*prime(n+1)-1)/2), print1(k", "))) \\ _Altug Alkan_, Nov 06 2015 %Y A086870 Cf. A103533, A120876. %K A086870 easy,nonn %O A086870 1,1 %A A086870 _Cino Hilliard_, Aug 20 2003