login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A118311 Number of dissimilar squarefree quaternary words of length n. 2
1, 1, 2, 4, 11, 29, 77, 202, 532, 1395, 3664, 9605, 25192, 66047, 173183, 453998, 1190259, 3120294, 8180124, 21444290, 56217025, 147373441, 386342414, 1012799936, 2655067412, 6960281083, 18246444362, 47833200849, 125395149294, 328724391241, 861753701567, 2259094233704 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,3
COMMENTS
Sherman Stein and A006156 count ordered squarefree(twin-free) ternary words. A060688 counts the dissimilar cases essentially by dividing by 3! (the number of ways to permute a,b,c). A051041 counts ordered squarefree quaternary words. A118311 counts the dissimilar cases (beginning with the 4th term) by dividing A051041 by 4!.
REFERENCES
Sherman Stein, How The Other Half Thinks, 2001, page 149.
LINKS
EXAMPLE
a(1) = 1 because a,b,c and d are similar.
a(2) = 1 because aa is not squarefree; so ab is the only valid case.
a(3) = 2 counting aba and abc.
a(4) = 4 counting abac, abca, abcb and abcd.
a(5) = 11 counting abaca,abacb,abcab,abcac,abcba,abacd,abcad,abcbd,abcda,abcdb and abcdc.
CROSSREFS
Sequence in context: A061860 A093960 A267912 * A132836 A148140 A328139
KEYWORD
nonn
AUTHOR
Alford Arnold, Apr 22 2006
EXTENSIONS
a(16)-a(25) from Max Alekseyev, Jul 03 2006
a(26)-a(30) from Giovanni Resta, Mar 20 2020
STATUS
approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 28 08:22 EDT 2024. Contains 371236 sequences. (Running on oeis4.)