A201015 Composite numbers whose product of digits is 2.

12, 21, 112, 121, 1112, 1121, 1211, 11112, 11121, 11211, 12111, 21111, 111112, 121111, 211111, 1111112, 1111121, 1112111, 1121111, 1211111, 2111111, 11111112, 11111121, 11111211, 11112111, 11121111, 11211111, 12111111, 21111111, 111111112, 111111121, 111111211

Offset

1,1

Comments

Complement of A107612 with respect to A199986. Subsequence of A199981 (composite numbers whose multiplicative digital root is 2).

Links

Michael S. Branicky, Table of n, a(n) for n = 1..10000

Example

Number 121 is in sequence because 1*2*1 = 2, and 121 = 11*11 is composite.

Mathematica

Select[Sort[Flatten[Table[FromDigits/@Permutations[PadRight[{2}, n, 1]], {n, 2, 9}]]], CompositeQ] (* Harvey P. Dale, Oct 06 2024 *)

Prog

(Python)
from sympy import isprime
def agen(maxdigits):
    for digs in range(1, maxdigits+1):
        for i in range(digs):
            t = int("1"*(digs-1-i) + "2" + "1"*i)
            if not isprime(t): yield t
print(list(agen(9))) # Michael S. Branicky, Dec 21 2021

Crossrefs

Cf. A107612 (primes whose product of digits is 2), A199986 (numbers whose product of digits is 2).

Sequence in context: A039402 A043225 A111066 * A044005 A321771 A225864

Adjacent sequences: A201012 A201013 A201014 * A201016 A201017 A201018

Keyword

nonn,base

Author

Jaroslav Krizek, Nov 25 2011

Status

approved