A201021 Composite numbers whose multiplicative digital root is 7.

117, 171, 711, 1711, 7111, 11711, 17111, 71111, 111117, 111171, 111711, 117111, 171111, 711111, 1111117, 1111171, 1711111, 7111111, 11111711, 11117111, 11171111, 11711111, 17111111, 111111117, 111111171, 111111711, 111117111, 111171111, 111711111, 117111111, 171111111

Offset: 1

Author

Jaroslav Krizek, Nov 25 2011

Keywords

• nonn
• base

Comments

Also composite numbers whose product of digits is 7.

Complement of A107693 with respect to A034054. ~

Crossrefs

Cf. A107693 (primes whose multiplicative digital root is 7), A034054 (numbers whose multiplicative digital root is 7).

Programs

Mathematica

Flatten[Table[Select[FromDigits/@Permutations[PadRight[{7},n,1]],CompositeQ],{n,9}]]//Sort (* Harvey P. Dale, Mar 29 2023 *)

Formula

Number 171 is in sequence because 1*7*1=7.