A199979 Nonprime numbers whose multiplicative digital root is 1.

1, 111, 1111, 11111, 111111, 1111111, 11111111, 111111111, 1111111111, 11111111111, 111111111111, 1111111111111, 11111111111111, 111111111111111, 1111111111111111, 11111111111111111, 111111111111111111, 11111111111111111111, 111111111111111111111

Offset: 1

Author

Jaroslav Krizek, Nov 13 2011

Keywords

• nonn
• base

Comments

1 and the composite repunits.

Number of 1's in a(n) is in A179005. Complement of A004022 with respect to A002275.

Examples

			1111 has a multiplicative digital root of 1 and it is the product of 11 and 101, therefore it is in the sequence.
11 has a multiplicative digital root of 1 but it is a prime number and therefore it is not in the sequence.
		

Crossrefs

Cf. A004022 (primes whose multiplicative digital root is 1).

Programs

Mathematica

Select[Table[(10^n - 1)/9, {n, 20}], Not[PrimeQ[#]] &] (* Alonso del Arte, Nov 23 2011 *)