The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A178127 Lesser of twin Ramanujan primes. 6

%I

%S 149,179,227,239,347,431,569,599,641,821,1019,1049,1061,1427,1487,

%T 1607,1787,1997,2081,2129,2237,2267,2657,2687,2711,2789,2999,3167,

%U 3257,3299,3359,3527,3539,3581,3671,3917,4091,4127,4229,4241,4337,4547,4637,4649

%N Lesser of twin Ramanujan primes.

%C By definition, a number p is a member if p and p+2 are Ramanujan primes A104272.

%C Conjecture: For all n > 570, more than 1/4 of the twin prime pairs < n are both Ramanujan primes.

%C Motivation for the conjecture is in "Ramanujan primes and Bertrand's postulate" Section 7.

%C Subsequence of A178128.

%H Amiram Eldar, <a href="/A178127/b178127.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..674 from R. J. Mathar)

%H B. Ghusayni, <a href="http://ijmcs.future-in-tech.net/7.2/R-Ghusayni-subsets.pdf">Subsets of prime numbers</a>, Int. J. Math. Comp. Sci. 7 (2) 2012

%H J. Sondow, <a href="http://arxiv.org/abs/0907.5232"> Ramanujan primes and Bertrand's postulate</a>, arXiv:0907.5232 [math.NT], 2009-2010.

%H J. Sondow, <a href="http://www.jstor.org/stable/40391170">Ramanujan primes and Bertrand's postulate</a>, Amer. Math. Monthly, 116 (2009), 630-635.

%H J. Sondow, J. W. Nicholson, and T. D. Noe, <a href="http://arxiv.org/abs/1105.2249"> Ramanujan Primes: Bounds, Runs, Twins, and Gaps</a>, J. Integer Seq. 14 (2011) Article 11.6.2.

%F {A104272(n): A104272(n+1) = A104272(n)+2}.

%F a(n) = A190654(2n-1) = A190654(2n) - 2.

%e a(1) = 149 because (149,151) is the 1st pair of twin primes both of which are Ramanujan primes.

%e 11 is not a member even though 11 and 13 are twin primes and 11 is a Ramanujan prime, because 13 is not also a Ramanujan prime.

%p n := 1:

%p for i from 1 do

%p if A104272(i+1) = A104272(i)+2 then

%p printf("%d %d\n",n,A104272(i)) ;

%p n := n+1 ;

%p end if;

%p end do: # produces b-file, _R. J. Mathar_, Sep 21 2017

%t nn = 1000; R = Table[0, {nn}]; s = 0;

%t Do[If[PrimeQ[k], s++]; If[PrimeQ[k/2], s--]; If[s < nn, R[[s + 1]] = k], {k, Prime[3*nn]}];

%t A104272 = R + 1;

%t twins1 = Position[A104272 // Differences, 2] // Flatten;

%t A104272[[twins1]] (* _Jean-François Alcover_, Oct 28 2018, after _T. D. Noe_ in A104272 *)

%o (Perl) use ntheory ":all"; my \$r = ramanujan_primes(1e5); my @rt = @\$r[grep { \$r->[\$_+1]-\$r->[\$_]==2 } 0..\$#\$r-1]; say "@rt"; # _Dana Jacobsen_, Sep 06 2015

%Y Cf. A001359, A104272, A164371, A178128, A190654.

%Y Cf. A181678 (number of twin Ramanujan prime pairs less than 10^n)

%K nonn

%O 1,1

%A _Jonathan Sondow_, May 20 2010

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified September 21 14:54 EDT 2020. Contains 337272 sequences. (Running on oeis4.)