A366921 a(n) is the least prime factor > 3 of 10^n - 1.

11, 37, 11, 41, 7, 239, 11, 37, 11, 21649, 7, 53, 11, 31, 11, 2071723, 7, 1111111111111111111, 11, 37, 11, 11111111111111111111111, 7, 41, 11, 37, 11, 3191, 7, 2791, 11, 37, 11, 41, 7, 2028119, 11, 37, 11, 83, 7, 173, 11, 31, 11, 35121409, 7, 239, 11, 37, 11, 107

Offset: 2

Author

Hugo Pfoertner, Oct 28 2023

Keywords

• nonn

Examples

			a(2) = 11 because 99 = 3^2 * 11 = 3^A366922(2) * A003020(2).
a(9) = 37 because 10^9 - 1 = 3^4 * 37 * 333667 = 3^A366922(9) * 37 * A003020(9).
		

Crossrefs

Cf. A002283, A003020, A102347, A366922.

Programs

PARI

a366921(n) = factor(10^n-1)[2,1]