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%I #45 Aug 29 2021 02:05:29
%S 1,121,1213121,121312141213121,1213121412131215121312141213121,
%T 121312141213121512131214121312161213121412131215121312141213121,
%U 1213121412131215121312141213121612131214121312151213121412131217121312141213121512131214121312161213121412131215121312141213121
%N Concatenation of terms of A018238.
%C Also called Zimin words.
%C a(n) is a palindrome for n<10; it is debatable whether a(n) can be called a Zimin word for n>=10 (see the Comments in A018238). - _Danny Rorabaugh_, Sep 26 2015
%H J. Cooper and D. Rorabaugh, <a href="http://arxiv.org/abs/1409.3080">Bounds on Zimin Word Avoidance</a>, arXiv:1409.3080 [math.CO], 2014; Congressus Numerantium, 222 (2014), 87-95.
%H L. J. Cummings and M. Mays, <a href="https://doi.org/10.37236/1571">A one-sided Zimin construction</a>, Electron. J. Combin. 8 (2001), #R27.
%H A. I. Zimin, <a href="http://mi.mathnet.ru/eng/msb2889">Blocking sets of terms</a>, Math. USSR Sbornik, 47 (1984), No. 2, 353-364.
%F The Zimin words are defined here by Z_1 = 1, Z_n = Z_{n-1}nZ_{n-1}. - _Dmitry Kamenetsky_, Sep 30 2006
%t a = {1}; Do[w = IntegerDigits@ a[[n - 1]]; AppendTo[a, FromDigits@ Join[w, IntegerDigits@ n, w]], {n, 2, 7}]; a (* _Michael De Vlieger_, Sep 26 2015 *)
%Y Cf. A018238, A123121, A033564.
%Y See A001511 for another representation of this sequence of digits.
%K base,nonn
%O 1,2
%A _Amarnath Murthy_, Apr 08 2003
%E More terms from _Joshua Zucker_, May 08 2006
%E "Palindromes" replaced with "Numbers" in sequence name by _Danny Rorabaugh_, Sep 26 2015
%E Shorter name by _Joerg Arndt_, Aug 28 2021
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