A068539 Number of quinternary cubefree words of length n.

1, 5, 25, 120, 580, 2800, 13500, 65100, 313940, 1513820, 7299760, 35200020, 169736780, 818482260, 3946776920, 19031623000

Offset

0,2

Links

Table of n, a(n) for n=0..15.

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 5*sum(cf("0"+"".join(w)) for w in product("01234", repeat=n-1))
print([a(n) for n in range(0, 10)]) # Michael S. Branicky, Apr 16 2021

Crossrefs

Cf. A028445, A051042, A051043.

Sequence in context: A269602 A089927 A269463 * A123871 A268453 A343801

Adjacent sequences: A068536 A068537 A068538 * A068540 A068541 A068542

Keyword

more,nonn

Author

Sascha Kurz, Mar 22 2002

Extensions

a(0) prepended and a(14)-a(15) by Michael S. Branicky, Apr 16 2021

Status

approved