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 A218561 #18 Feb 13 2013 23:58:31 %S A218561 29,71,137,197,239,269,347,419,431,641,659,809,821,1061,1091,1151, %T A218561 1289,1489,1607,1721,1783,1877,1949,1993,2083,2141,2267,2339,2381, %U A218561 2389,2549,2729,2801,2833,2969,2999,3019,3041,3217,3253,3299,3329,3389,3461 %N A218561 4-gap primes: Prime p is a term iff there is no prime between 4*p and 4*nextprime(p), where nextprime=A151800. %H A218561 M. F. Hasler, <a href="/A218561/b218561.txt">Table of n, a(n) for n = 1..4911</a> (all terms up to 5*10^5). %e A218561 29 is in the sequence since there are no primes in the interval(4*29,4*31)=(116,124) %t A218561 Select[Prime[Range[500]],Count[Range[4*#,4*NextPrime[#]],_?PrimeQ]==0&] (* _Harvey P. Dale_, Jan 05 2013 *) %o A218561 (PARI) is_A218561(p)=isprime(p)&nextprime(4*p)>=4*nextprime(p+1) \\ - _M. F. Hasler_, Nov 03 2012 %Y A218561 Cf. A195270, A195271, A195377, A195325, A195329, A195379. %K A218561 nonn %O A218561 1,1 %A A218561 _Vladimir Shevelev_ and _Peter J. C. Moses_, Nov 02 2012