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

7, 19, 989, 1187, 4183, 19607

Offset

1,1

Comments

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

Links

Table of n, a(n) for n=1..6.

Example

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

Mathematica

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

Crossrefs

Cf. A267327.

Sequence in context: A359636 A330875 A330852 * A057866 A329001 A334982

Adjacent sequences: A267234 A267235 A267236 * A267238 A267239 A267240

Keyword

nonn,more,hard

Author

Mikk Heidemaa, Jan 12 2016

Extensions

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

Status

approved