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 A267237 #22 Nov 15 2024 14:55:42
%S A267237 7,19,989,1187,4183,19607
%N A267237 Numbers k such that 'k-1' prepended to 'k' k times yield primes.
%C A267237 a(5) produces the prime: 4182...[4183 times]4183 (16736 digits).
%e A267237 7 occurs because '6' prepended 7 times to '7' produces prime (66666667).
%t A267237 Select[ Range[1200], PrimeQ[ FromDigits[ StringJoin@@ ToString/@ PadLeft[{#}, #+1, #-1]]] &]
%Y A267237 Cf. A267327.
%K A267237 nonn,more,hard
%O A267237 1,1
%A A267237 _Mikk Heidemaa_, Jan 12 2016
%E A267237 a(6) from _Michael S. Branicky_, Nov 15 2024