|
%I #21 Feb 18 2024 05:32:44
%S 2,4,8,16,32,52,100,160,260,424,684,988,1588,2342,3458,5072,7516,
%T 10546,15506,21496,30682,42508,60170,81316,114182,153768,212966,
%U 283502,390168,513652
%N Number of length-n binary words w such that ww is rich.
%C A rich word w is one that contains, as contiguous subwords, exactly n nonempty palindromes, where n is the length of w. An infinite word is rich if all of its (contiguous) subwords are rich. By a theorem of Glen, Justin, Widmer, and Zamboni (below), a(n) is also the number of length-n binary words w such that the infinite word www... is rich. And also the number of length-n binary words w that are products of two palindromes, where all the conjugates of w are rich.
%H A. Glen, J. Justin, S. Widmer, and L. Q. Zamboni, <a href="https://doi.org/10.1016/j.ejc.2008.04.006">Palindromic richness</a>, European J. Combinatorics 30 (2009), 510-531. See Theorem 3.1, p. 515.
%o (Python)
%o from itertools import product
%o def pal(w): return w == w[::-1]
%o def rich(w):
%o subs = (w[i:j] for i in range(len(w)) for j in range(i+1, len(w)+1))
%o return len(w) == sum(pal(s) for s in set(subs))
%o def a(n):
%o binn = ("0"+"".join(b) for b in product("01", repeat=n-1))
%o return sum(2 for w in binn if rich(w+w))
%o print([a(n) for n in range(1, 16)]) # _Michael S. Branicky_, Jul 07 2022
%Y Cf. A216264.
%K nonn,more
%O 1,1
%A _Jeffrey Shallit_, Feb 06 2019
%E a(17)-a(30) from _Lars Blomberg_, Feb 13 2019
|
