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

1,2

Comments

1 and the composite repunits.

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

Links

Table of n, a(n) for n=1..19.

Example

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.

Mathematica

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

Crossrefs

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

Sequence in context: A289964 A290619 A290627 * A102782 A302475 A303256

Adjacent sequences: A199976 A199977 A199978 * A199980 A199981 A199982

Keyword

nonn,base

Author

Jaroslav Krizek, Nov 13 2011

Status

approved