OFFSET
0,2
LINKS
Eric Weisstein's World of Mathematics, Cubefree Word.
EXAMPLE
a(3) counts the 64 three-letter words minus 000, 111, 222, and 333.
PROG
(Python)
from itertools import product
def cf(s):
for l in range(1, len(s)//3+1):
for i in range(len(s) - 3*l+1):
if s[i:i+l]*2 == s[i+l:i+3*l]: return False
return True
def a(n):
if n == 0: return 1
return 4*sum(cf("0"+"".join(w)) for w in product("0123", repeat=n-1))
print([a(n) for n in range(1, 12)]) # Michael S. Branicky, Apr 16 2021
CROSSREFS
KEYWORD
nonn,more
AUTHOR
EXTENSIONS
More terms from Sascha Kurz, Mar 22 2002
a(0) prepended and a(16)-a(17) from Michael S. Branicky, Apr 16 2021
STATUS
approved