login
A086870
Primes equal to a product of twin primes minus 1 divided by 2.
6
7, 17, 71, 449, 881, 2591, 9521, 39761, 106721, 179999, 206081, 342791, 388961, 596231, 847601, 1292831, 2268449, 2571911, 2836961, 3612671, 6223391, 6329681, 6415361, 8520191, 8946449, 9409121, 10342151, 12550049, 16485281, 18800711
OFFSET
1,1
COMMENTS
From Jason Kimberley, Oct 22 2015 (Start)
Prime elements of A120876.
For each p in this list, A001221(2p) = A001222(2p) = A001221(2p+1) = A001222(2p+1) = 2.
2*a(n) is a subsequence of A103533. They first differ when 313619 is not in this sequence, but 2*313619 = 627238 = A103533(12).
(End)
LINKS
FORMULA
Primes of the form (t1*t2-1)/2, where t1, t2 are twin primes.
EXAMPLE
t1 = 71,t2 = 73, (71*73-1)/2 = 5182/2 = 2591 = prime.
MATHEMATICA
Select[(Times[#, # + 2] - 1)/2 &@ Select[Prime@ Range@ 1000, PrimeQ[# + 2] &], PrimeQ] (* Michael De Vlieger, Nov 06 2015 *)
PROG
(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
CROSSREFS
Sequence in context: A034054 A120876 A216073 * A107693 A217717 A122528
KEYWORD
easy,nonn
AUTHOR
Cino Hilliard, Aug 20 2003
STATUS
approved