A201016 Composite numbers whose product of digits is 4.

4, 14, 22, 114, 122, 141, 212, 221, 411, 1114, 1122, 1141, 1212, 1221, 1411, 2112, 2121, 2211, 11114, 11122, 11141, 11212, 11221, 12112, 12121, 14111, 21112, 41111, 111114, 111122, 111141, 111212, 111221, 111411, 112112, 112211, 114111, 121112, 121121, 121211, 122111

Offset

1,1

Comments

Complement of A107690 with respect to A199987. Subsequence of A199983 (composite numbers whose multiplicative digital root is 4).

Links

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Example

Number 122 is in sequence because 1*2*2=4.

Mathematica

Select[Range[125000], CompositeQ[#]&&Times@@IntegerDigits[#]==4&] (* Harvey P. Dale, May 15 2025 *)
(* Alternative: *)
Module[{nn=6, f, t}, f=Flatten[Table[Select[FromDigits/@Permutations[PadRight[{4}, d, 1]], CompositeQ], {d, nn}]]; t=Flatten[Table[Select[FromDigits/@Permutations[PadRight[{2, 2}, d, 1]], CompositeQ], {d, nn}]]; Join[f, t]]//Sort (* The second program is much faster than the first. *) (* Harvey P. Dale, May 15 2025 *)

Crossrefs

Cf. A107690 (primes whose product of digits is 4), A199987 (numbers whose product of digits is 4).

Sequence in context: A298008 A279414 A199987 * A317734 A154046 A000054

Adjacent sequences: A201013 A201014 A201015 * A201017 A201018 A201019

Keyword

nonn,base

Author

Jaroslav Krizek, Nov 25 2011

Status

approved