A381040 Numbers k such that the concatenation of 1, k! and 1 is prime.

7, 9, 10, 15, 21225

Offset

1,1

Comments

Also, numbers k such that 10^(A034886(k)+1) + 10k! + 1 is prime.

For k >= 5, the foregoing requires that 10^(A034886(k)+1) + 1 has no prime factors <= k.

a(6) > 10^5. - Michael S. Branicky, Apr 24 2025

Links

Table of n, a(n) for n=1..5.

Example

The concatenation of 1, 7! and 1 is 150401, which is prime, so 7 is a term.

Crossrefs

Cf. A000142, A034886.

Corresponding primes are in A262195.

Sequence in context: A386707 A118621 A117933 * A112162 A058483 A176326

Adjacent sequences: A381037 A381038 A381039 * A381041 A381042 A381043

Keyword

nonn,base

Author

Michael S. Branicky, Apr 14 2025

Status

approved