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

Author

Jaroslav Krizek, Nov 25 2011

Keywords

• nonn
• base

Comments

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

Examples

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

Links

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

Crossrefs

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

Programs

Mathematica

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

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