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

1,1

Links

Table of n, a(n) for n=1..59.

Formula

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

Example

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

Prog

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

Crossrefs

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

Sequence in context: A152075 A092789 A092793 * A285130 A024909 A063241

Adjacent sequences: A361974 A361975 A361976 * A361978 A361979 A361980

Keyword

nonn,easy

Author

Jack Braxton, Apr 02 2023

Status

approved