A358684 a(n) is the minimum integer k such that the smallest prime factor of the n-th Fermat number exceeds 2^(2^n - k).

0, 0, 0, 0, 0, 23, 46, 73, 206, 491, 999, 2030, 4080, 8151

Offset: 0

Author

Lorenzo Sauras Altuzarra, Nov 26 2022

Keywords

• nonn
• more

Comments

2^(2^n - a(n)) < A093179(n).

Conjecture: the dyadic valuation of A093179(n) - 1 does not exceed 2^n - a(n).

a(14) is probably equal to 16208; a(15) to a(19) are 32738, 65507, 131028, 262121, 524252; a(20) is unknown; a(21) to a(23) are 2097110, 4194189, 8388581; a(24) is unknown.

Examples

			For n=5, the smallest prime factor of F(5) = 2^(2^5) + 1 is 641 and it falls between 2^(2^5 - 23) = 512 < 641 < 1024 = 2^(2^5 - 22) so that a(5) = 23.
		

Links

• Lorenzo Sauras-Altuzarra, Some properties of the factors of Fermat numbers, Art Discrete Appl. Math. (2022).

Crossrefs

Cf. A000215, A093179.

Formula

Conjecture: a(n) ~ 2^n as n -> oo.