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 A023270 #21 Sep 08 2022 08:44:47 %S A023270 5,13,19,23,47,61,103,127,139,233,257,293,331,433,463,491,499,547,593, %T A023270 701,859,967,1009,1013,1049,1223,1321,1399,1493,1567,1723,1811,1867, %U A023270 1889,1997,2039,2063,2087,2137,2203,2267,2273,2309,2423,2437,2441,2609,2633 %N A023270 Primes that remain prime through 2 iterations of function f(x) = 10x + 9. %C A023270 Primes p such that 10*p+9 and 100*p+99 are also primes. - _Vincenzo Librandi_, Aug 04 2010 %H A023270 Harvey P. Dale, <a href="/A023270/b023270.txt">Table of n, a(n) for n = 1..1000</a> %t A023270 Select[Prime[Range[400]],AllTrue[NestList[10#+9&,#,2],PrimeQ]&] (* The program uses the AllTrue function from Mathematica version 10 *) (* _Harvey P. Dale_, Jun 05 2015 *) %o A023270 (Magma) [n: n in [1..100000] | IsPrime(n) and IsPrime(10*n+9) and IsPrime(100*n+99)] // _Vincenzo Librandi_, Aug 04 2010 %K A023270 nonn %O A023270 1,1 %A A023270 _David W. Wilson_