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%I #27 Dec 21 2021 10:46:46
%S 12,21,1122,1212,1221,2112,2121,2211,111222,112122,112212,112221,
%T 121122,121212,121221,122112,122121,122211,211122,211212,211221,
%U 212112,212121,212211,221112,221121,221211,222111,11112222,11121222,11122122,11122212,11122221
%N List of words over {1,2} with equal numbers of 1's and 2's.
%C Of course the empty word also has this property.
%C All of these, interpreted as decimal integers are divisible by 3, because each pair of "1" and "2" contributes a digital sum of 3, hence the total is divisible by 3. Is there a semiprime in the sequence after 21? - _Jonathan Vos Post_, Jul 18 2012
%C The semiprime subsequence contains 21, 11222121, 12122211, 21221121, 22211121, 22212111, and continues with 14 10-digit entries etc. - _R. J. Mathar_, Jul 19 2012
%D J.-P. Allouche and J. Shallit, Automatic Sequences, Cambridge Univ. Press, 2003, p. 2.
%H Harvey P. Dale, <a href="/A214218/b214218.txt">Table of n, a(n) for n = 1..1000</a>
%p sort([seq(seq((10^(2*d)-1)/9+add(10^i,i=s),s=combinat:-choose([$0..(2*d-1)],d)),d=1..4)]); # _Robert Israel_, Jan 02 2018
%t Sort[FromDigits/@Flatten[Table[Permutations[PadRight[{},2n,{1,2}]],{n,3}],1]] (* _Harvey P. Dale_, Aug 30 2016 *)
%o (Python)
%o from itertools import count, islice
%o from sympy.utilities.iterables import multiset_permutations as mp
%o def agen():
%o for d in count(2, 2):
%o for s in mp("1"*(d//2) + "2"*(d//2), d):
%o yield int("".join(s))
%o print(list(islice(agen(), 33))) # _Michael S. Branicky_, Dec 21 2021
%Y Subsequence of A007931, A111066.
%K nonn,base
%O 1,1
%A _N. J. A. Sloane_, Jul 18 2012