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 A087679 Numbers k such that both k+2 and k-2 are prime. 18
 5, 9, 15, 21, 39, 45, 69, 81, 99, 105, 111, 129, 165, 195, 225, 231, 279, 309, 315, 351, 381, 399, 441, 459, 465, 489, 501, 615, 645, 675, 741, 759, 771, 825, 855, 861, 879, 885, 909, 939, 969, 1011, 1089, 1095, 1215, 1281, 1299, 1305, 1425, 1431, 1449, 1485 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Essentially the same as A029708: a(n) = A029708(n-1) for n>=2. Midpoint of cousin prime pairs. The only prime is 5. All other terms are multiples of 3. - Zak Seidov, May 19 2014 LINKS M. F. Hasler, Table of n, a(n) for n = 1..1000 Eric Weisstein's World of Mathematics, Cousin Primes FORMULA a(n) = (A023200(n) + A046132(n))/2 = A023200(n) + 2 = A046132(n) - 2. a(n+1) = A056956(n)*6 + 3 = A157834(n)*3; a(n) = A088762(n)*2 + 1. - M. F. Hasler, Apr 05 2017 MAPLE ZL:=[]:for p from 1 to 1485 do if (isprime(p) and isprime(p+4) ) then ZL:=[op(ZL), (p+(p+4))/2]; fi; od; print(ZL); # Zerinvary Lajos, Mar 07 2007 MATHEMATICA lst={}; Do[If[PrimeQ[n-2]&&PrimeQ[n+2], AppendTo[lst, n]], {n, 3, 8!, 2}]; lst (* Vladimir Joseph Stephan Orlovsky, Jan 14 2009 *) PROG (PARI) s=[]; for(n=1, 2000, if(isprime(n-2) && isprime(n+2), s=concat(s, n))); s \\ Colin Barker, May 19 2014 (PARI) is_A087679(n)={isprime(n-2) && isprime(n+2)} \\ For numbers >> 10^12 one should add conditions {n%6==3 && ... || n==5} or consider only such numbers congruent to 3 (mod 6). - M. F. Hasler, Apr 05 2017 CROSSREFS Cf. A014574, A087678-A087683, A087695-A087697, A088762. Sequence in context: A315078 A315079 A217277 * A179187 A315080 A315081 Adjacent sequences:  A087676 A087677 A087678 * A087680 A087681 A087682 KEYWORD nonn,easy AUTHOR Zak Seidov, Sep 27 2003 EXTENSIONS More terms from Ray Chandler, Oct 26 2003 STATUS approved

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Last modified September 26 08:01 EDT 2020. Contains 337346 sequences. (Running on oeis4.)