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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 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.)