A267237 Numbers k such that 'k-1' prepended to 'k' k times yield primes.

7, 19, 989, 1187, 4183, 19607

Offset: 1

Author

Mikk Heidemaa, Jan 12 2016

Keywords

• nonn
• more
• hard

Comments

a(5) produces the prime: 4182...[4183 times]4183 (16736 digits).

Examples

			7 occurs because '6' prepended 7 times to '7' produces prime (66666667).
		

Crossrefs

Cf. A267327.

Programs

Mathematica

Select[ Range[1200], PrimeQ[ FromDigits[ StringJoin@@ ToString/@ PadLeft[{#}, #+1, #-1]]] &]

Extensions

a(6) from Michael S. Branicky, Nov 15 2024