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A201015 Composite numbers whose product of digits is 2.

%I A201015 #15 Oct 06 2024 17:19:59
%S A201015 12,21,112,121,1112,1121,1211,11112,11121,11211,12111,21111,111112,
%T A201015 121111,211111,1111112,1111121,1112111,1121111,1211111,2111111,
%U A201015 11111112,11111121,11111211,11112111,11121111,11211111,12111111,21111111,111111112,111111121,111111211
%N A201015 Composite numbers whose product of digits is 2.
%C A201015 Complement of A107612 with respect to A199986. Subsequence of A199981 (composite numbers whose multiplicative digital root is 2).
%H A201015 Michael S. Branicky, <a href="/A201015/b201015.txt">Table of n, a(n) for n = 1..10000</a>
%e A201015 Number 121 is in sequence because 1*2*1 = 2, and 121 = 11*11 is composite.
%t A201015 Select[Sort[Flatten[Table[FromDigits/@Permutations[PadRight[{2},n,1]],{n,2,9}]]],CompositeQ] (* _Harvey P. Dale_, Oct 06 2024 *)
%o A201015 (Python)
%o A201015 from sympy import isprime
%o A201015 def agen(maxdigits):
%o A201015     for digs in range(1, maxdigits+1):
%o A201015         for i in range(digs):
%o A201015             t = int("1"*(digs-1-i) + "2" + "1"*i)
%o A201015             if not isprime(t): yield t
%o A201015 print(list(agen(9))) # _Michael S. Branicky_, Dec 21 2021
%Y A201015 Cf. A107612 (primes whose product of digits is 2), A199986 (numbers whose product of digits is 2).
%K A201015 nonn,base
%O A201015 1,1
%A A201015 _Jaroslav Krizek_, Nov 25 2011