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 A309607 #32 Sep 02 2019 15:04:03 %S A309607 2,3,5,53,189,293,1107,2181,2695,2871,7667,19999,27471,44537,62323, %T A309607 134367,174295 %N A309607 Exponents k for which reversal(2^k-1) is prime. %C A309607 According to the statements in the first link given below, the terms for k <= 7667, the primes of the form reversal(2^k-1) are certified and for k >= 19999 they are probable primes. %H A309607 Carlos Rivera, <a href="https://www.primepuzzles.net/puzzles/puzz_417.htm">Puzzle 417</a>. %H A309607 Chris K. Caldwell and G. L. Honaker, <a href="https://primes.utm.edu/curios/page.php?number_id=7267&submitter=Rivera">Prime Curio for 1990474529917009</a>. %e A309607 5 is included because for n=5, reversal(2^5-1)=13 is prime. %t A309607 Select[Range[7667], PrimeQ[IntegerReverse[2^# - 1]] &] %o A309607 (PARI) isok(k) = isprime(fromdigits(Vecrev(digits(2^k-1)))); \\ _Michel Marcus_, Aug 10 2019 %Y A309607 Cf. A000225, A000668, A001348, A000079, A000043, A057708, A134037, A134038, A134039. %K A309607 nonn,base,more %O A309607 1,1 %A A309607 _Metin Sariyar_, Aug 09 2019