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 A226993 #14 Jun 30 2013 03:03:44 %S A226993 2,3,7,11,17,19,29,43,47,53,71,79,83,89,97,101,107,109,127,151,173, %T A226993 179,191,197,199,223,227,233,241,251,271,277,281,307,317,331,349,353, %U A226993 359,379,389,397,431,439,443,449,457,461,467,479,503,521,523,547,557,569 %N A226993 Primes p such that 2p - 1 and 2p + 1 are squarefree. %H A226993 Paolo P. Lava, <a href="/A226993/b226993.txt">Table of n, a(n) for n = 1..1000</a> %e A226993 3 is in the sequence because 3 is prime and both 2*3 - 1 = 5 and 2*3 + 1 = 7 are squarefree. %p A226993 with(numtheory); P:=proc(q) local n; for n from 1 to q do %p A226993 if isprime(n) then if issqrfree(2*n-1) and issqrfree(2*n+1) then print(n) %p A226993 fi; fi; od; end: P(10^5); #_Paolo P. Lava_, Jun 26 2013 %o A226993 (PARI) is(n)=isprime(n)&&issquarefree(2*n-1)&&issquarefree(2*n+1) \\ _Charles R Greathouse IV_, Jun 27 2013 %Y A226993 Cf. A005117. %K A226993 nonn %O A226993 1,1 %A A226993 _Irina Gerasimova_, Jun 26 2013