A201021 - OEIS

%I #13 Sep 22 2025 08:57:21

%S 117,171,711,1711,7111,11711,17111,71111,111117,111171,111711,117111,

%T 171111,711111,1111117,1111171,1711111,7111111,11111711,11117111,

%U 11171111,11711111,17111111,111111117,111111171,111111711,111117111,111171111,111711111,117111111,171111111

%N Composite numbers whose multiplicative digital root is 7.

%C Also composite numbers whose product of digits is 7.

%C Complement of A107693 with respect to A034054. ~

%e 171 is a term because 1*7*1=7.

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

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

%K nonn,base

%O 1,1

%A _Jaroslav Krizek_, Nov 25 2011