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 A050717 #9 Oct 15 2019 18:17:40 %S A050717 13,19,21,27,33,39,49,51,57,63,67,73,87,91,97,107,137,141,147,153,159, %T A050717 191,197,203,207,219,221,227,249,263,273,279,311,323,327,339,351,353, %U A050717 359,381,389,429,477,479,497,503,507,513,519,521,533,551,569,573,593 %N A050717 Inserting a digit '7' between adjacent digits of n makes a prime. %e A050717 E.g. 477 becomes 4(7)7(7)7 which is prime 47777. %t A050717 Select[Range[10, 500], PrimeQ[FromDigits[Riffle[IntegerDigits[#], 7]]]&] (* _Georg Fischer_, Oct 15 2019 after Harvey P. Dale *) %Y A050717 Cf. A050674, A050711, A050712, A050713, A050714, A050716, A050717, A050718, A050719. %K A050717 nonn,base %O A050717 1,1 %A A050717 _Patrick De Geest_, Aug 15 1999 %E A050717 Offset changed to 1 by _Georg Fischer_, Oct 15 2019