Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #18 Nov 06 2017 02:48:12
%S 3,5,13,17,29,31,43,67,71,97,107,109,131,157,181,191,223,233,239,269,
%T 281,313,359,379,383,401,409,431,503,523,569,571,619,631,659,691,719,
%U 751,787,797,857,859,881,883,971,1039,1061,1063,1091,1117,1123,1201
%N Primes which occur exactly twice in the sequence of a(n) = prime(n) - prime(n - pi(n)) = A065308(n).
%C In A065308, each odd prime seems to appear once or twice.
%H Harry J. Smith, <a href="/A065311/b065311.txt">Table of n, a(n) for n = 1..1000</a>
%t Most@ Select[Tally@ Array[Prime[# - PrimePi@ #] &, 300], Last@ # == 2 &][[All, 1]] (* _Michael De Vlieger_, Nov 03 2017 *)
%o (PARI) { n=0; p=1; f=2; m=1; for (i=1, 10^9, a=0; p=nextprime(p + 1); while (p==f, a++; m++; f=prime(m - primepi(m))); if (a==2, write("b065311.txt", n++, " ", p); if (n==1000, return)) ) } \\ _Harry J. Smith_, Oct 16 2009
%Y Cf. A000040, A000720, A054546, A065308, A065309, A065310, A065311.
%K nonn
%O 1,1
%A _Labos Elemer_, Oct 29 2001