OFFSET
0,2
COMMENTS
A word w is "privileged" if it is of length <= 1, or if it has a privileged prefix that appears exactly twice in w, once as a prefix and once as a suffix (which may overlap).
All terms beyond a(0) are even because the 1's complement of a privileged word is again privileged (and different). - M. F. Hasler, Nov 05 2013
LINKS
Gabriele Fici, Open and Closed Words, in Giovanni Pighizzini, ed., The Formal Language Theory Column, Bulletin of EATCS, 2017.
M. Forsyth, A. Jayakumar, and J. Shallit, Remarks on Privileged Words, arXiv preprint arXiv:1311.7403 [cs.FL], 2013.
Daniel Gabric, Asymptotic bounds for the number of closed and privileged words, arXiv:2206.14273 [math.CO], 2022.
J. Peltomäki, Introducing privileged words: privileged complexity of Sturmian words, Theoret. Comput. Sci. 500 (2013), 57-67.
J. Peltomäki, Privileged factors in the Thue-Morse word — a comparison of privileged words and palindromes, arXiv:1306.6768 [math.CO], 2013-2015.
J. Peltomäki, Privileged Words and Sturmian Words, Ph.D. Dissertation, TUCS Dissertations 214, 2016.
EXAMPLE
For n = 5 the privileged words are {00000,00100,01010,01110,10001,10101,11011,11111}.
See A235609 for the full list of privileged words.
The least non-palindromic privileged word is 00101100, of length 8. - M. F. Hasler, Nov 05 2013
PROG
(PARI) A231208=n->{local(isp(w, n, p)=setsearch([0, 2^(n-1)-2, 2^(n-1)+1, 2^n-1], w)&&return(1); for(i=1, n-2, (w-p=w>>i)%2^(n-i)&&next; for(j=1, i-1, (w>>j-p)%2^(n-i)||next(2)); isp(p, n-i)&&return(1))); sum(i=1, 2^(n-1)-1, isp(i, n), 1)*2-!n} \\ M. F. Hasler, Nov 05 2013
(Python)
from itertools import count, islice, product
def comp(w): return "".join("2" if c == "1" else "1" for c in w)
def agen():
prev, priv = 0, set("1"); yield 1
for d in count(2):
yield 2*(len(priv) - prev)
prev = len(priv)
for p in product("12", repeat=d-1):
w, passes = "1" + "".join(p), False
if len(set(w)) == 1: passes = True
elif len(w.lstrip(w[0])) != len(w.rstrip(w[0])): passes = False
else:
for i in range(1, len(w)):
p, s = w[:i], w[-i:]
if p == s and p not in w[1:-1] and p in priv:
passes = True; break
if passes: priv.add(w)
print(list(islice(agen(), 20))) # Michael S. Branicky, Jul 01 2022
CROSSREFS
KEYWORD
nonn
AUTHOR
Jeffrey Shallit, Nov 05 2013
EXTENSIONS
Terms a(22) to a(30) computed by Michael Forsyth
More terms from Forsyth et al. (2013) added by N. J. A. Sloane, Jan 23 2014
Terms a(39)-a(45) from Peltomäki's dissertation (2016) added by Jarkko Peltomäki, Aug 24 2016
STATUS
approved