The OEIS is supported by the many generous donors to the OEIS Foundation.
Reminder: The OEIS is hiring a new managing editor, and the application deadline is January 26.
Reminder: The OEIS is hiring a new managing editor, and the application deadline is January 26.
%I #22 Nov 15 2024 14:55:42
%S 7,19,989,1187,4183,19607
%N Numbers k such that 'k-1' prepended to 'k' k times yield primes.
%C a(5) produces the prime: 4182...[4183 times]4183 (16736 digits).
%e 7 occurs because '6' prepended 7 times to '7' produces prime (66666667).
%t Select[ Range[1200], PrimeQ[ FromDigits[ StringJoin@@ ToString/@ PadLeft[{#}, #+1, #-1]]] &]
%Y Cf. A267327.
%K nonn,more,hard
%O 1,1
%A _Mikk Heidemaa_, Jan 12 2016
%E a(6) from _Michael S. Branicky_, Nov 15 2024