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 A117242 #16 Sep 27 2024 16:33:30
%S A117242 2,23,37,47,53,67,83,89,113,127,131,157,167,211,233,251,257,263,293,
%T A117242 307,317,337,353,359,379,389,401,409,443,449,467,479,487,491,499,503,
%U A117242 509,541,557,563,577,587,631,647,653,677,683,701,719,743,751,761,769,787,797,839,863,877,887,911,919,937,941,947,953,971,977,983,991
%N A117242 Chen primes that are not twin primes.
%H A117242 Charles R Greathouse IV, <a href="/A117242/b117242.txt">Table of n, a(n) for n = 1..10000</a>
%e A117242 a(1) = 2, 2 is a Chen prime but is not in a twin prime pair.
%e A117242 a(2) = 23 is a Chen prime, but is not in a twin prime pair.
%p A117242 ischenprime:=proc(n); if (isprime(n) = 'true') then if (isprime(n+2) = 'true' or numtheory[bigomega](n+2) = 2) then RETURN('true') else RETURN('false') fi fi end:
%p A117242 ts_chen_non_twin_primes:=proc(n) local i, ans; ans:=[ ]: for i from 1 to n do if (ischenprime(i) = 'true') and (isprime(i+2) = 'false' and isprime(i-2) = 'false') then ans:=[op(ans), i]: fi od; RETURN(ans) end:
%p A117242 ts_chen_non_twin_primes(1000);
%t A117242 Lim=PrimePi[1000];Select[Select[Prime[Range[Lim]],PrimeOmega[#+2]<3&],!MemberQ[Select[ Prime[ Range[Lim]], PrimeQ[ # - 2]||PrimeQ[#+2]&] ,#]&] (* _James C. McMahon_, Sep 27 2024 *)
%o A117242 (PARI) is(n)=isprime(n)&&bigomega(n+2)==2&&!isprime(n-2) \\ _Charles R Greathouse IV_, May 04 2013
%Y A117242 Cf. A001097, A109611.
%K A117242 nonn
%O A117242 1,1
%A A117242 _Jani Melik_, Apr 22 2006