A361977 a(n) is the largest prime p such that 2^p - 1 <= 10^n.

3, 5, 7, 13, 13, 19, 23, 23, 29, 31, 31, 37, 43, 43, 47, 53, 53, 59, 61, 61, 67, 73, 73, 79, 83, 83, 89, 89, 89, 97, 101, 103, 109, 109, 113, 113, 113, 113, 127, 131, 131, 139, 139, 139, 149, 151, 151, 157, 157, 163, 167, 167, 173, 179, 181, 181, 181, 191, 193

Offset: 1

Author

Jack Braxton, Apr 02 2023

Keywords

• nonn
• easy

Examples

			a(5) = 13, 2^13 - 1 = 8192 < 10^5.
		

Crossrefs

Cf. A000040, A000043, A000668, A011557, A067497, A151799.

Programs

Python

from sympy import prevprime
def A361977(n): return prevprime((10**n-1).bit_length()) # Chai Wah Wu, Apr 02 2023

Formula

a(n) = A151799(A067497(n)). - Alois P. Heinz, Apr 02 2023