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 A260801 #15 Sep 22 2015 05:09:24 %S A260801 2,7,17,29,31,71,89,107,113,127,131,157,181,223,239,263,271,277,281, %T A260801 283,313,337,379,409,419,421,431,503,547,571,577,691,701,727,757,809, %U A260801 821,857,883,947,953,971,1031,1109,1129,1153,1163,1231,1283,1291,1327,1361,1447,1487,1531,1559,1567,1583 %N A260801 Primes p such that A008908(p) is also prime. %H A260801 <a href="/index/3#3x1">Index entries for sequences related to 3x+1 (or Collatz) problem</a> %e A260801 7 is prime and A008908(7) is 17, which is also prime. Therefore, 7 is in the sequence. %t A260801 collatz[n_]:=If[EvenQ[n],n/2,3*n+1]; %t A260801 seq[n_]:=Length[NestWhileList[collatz,n,#!= 1&]] %t A260801 Select[Flatten[Position[seq/@Range[7!],_?PrimeQ]],PrimeQ] %o A260801 (PARI) T(n)=c=0;while(n!=1,if(n%2,n=3*n+1;c++);if(!(n%2),n=n/2;c++));c+1 %o A260801 forprime(p=1,10^3,if(isprime(T(p)),print1(p,", "))) \\ _Derek Orr_, Aug 27 2015 %Y A260801 Cf. A008908. %K A260801 easy,nonn %O A260801 1,1 %A A260801 _Ivan N. Ianakiev_, Jul 31 2015