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A199979 Nonprime numbers whose multiplicative digital root is 1. 0
1, 111, 1111, 11111, 111111, 1111111, 11111111, 111111111, 1111111111, 11111111111, 111111111111, 1111111111111, 11111111111111, 111111111111111, 1111111111111111, 11111111111111111, 111111111111111111, 11111111111111111111, 111111111111111111111 (list; graph; refs; listen; history; text; internal format)
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

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Last modified March 20 22:57 EDT 2023. Contains 361392 sequences. (Running on oeis4.)