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 A262769 #37 Jul 07 2024 01:32:15 %S A262769 3,23,63,261,281,291,4689,10641,11231,12519 %N A262769 Integers k such that the concatenation of 2^k and k is prime. %C A262769 First three primes: 83, 838860823, 922337203685477580863. %C A262769 a(11) > 120000. - _Giovanni Resta_, Apr 08 2016 %C A262769 a(11) > 160000. - _Michael S. Branicky_, Jul 06 2024 %e A262769 For k = 23 we have 2^23 and 23 equal to 8388608 and 23, respectively, and 838860823 is a prime number. %t A262769 Select[Range@ 5000, PrimeQ[2^# * 10^IntegerLength[#] + #] &] (* _Giovanni Resta_, Apr 08 2016 *) %o A262769 (Python) %o A262769 from sympy import isprime %o A262769 def afind(limit): %o A262769 k, twok = 0, 1 %o A262769 while k <= limit: %o A262769 if isprime(int(str(twok) + str(k))): print(k, end = ", ") %o A262769 k, twok = k+1, twok*2 %o A262769 afind(2000) # _Michael S. Branicky_, Mar 23 2021 %o A262769 (PARI) isok(k) = isprime(eval(Str(2^k, k))); \\ _Michel Marcus_, Mar 23 2021 %Y A262769 Cf. A000079. %K A262769 nonn,base,more %O A262769 1,1 %A A262769 _Emre APARI_, Mar 24 2016 %E A262769 a(9)-a(10) from _Giovanni Resta_, Apr 08 2016 %E A262769 a(8) inserted by _Michael S. Branicky_, Jul 06 2024