OFFSET
1,24
COMMENTS
How many ways are of arranging the numbers 1,1,1,1,2,2,2,2,3,3,3,3,...,n,n,n,n so that there are zero numbers between the first and second 1's, between the second and third 1's and between the third and fourth 1's; one number between the first and second 2's, between the second and third 2's and between the third and fourth 2's; ... n-1 numbers between the first and second n's, between the second and third n's and between the third and fourth n's?
An equivalent definition is A261517 with added condition that all different common intervals are <= n.
a(n) ignores reflected solutions.
LINKS
Fausto A. C. Cariboni, Solutions for a(24)-a(25)
J. E. Miller, Langford's Problem.
FORMULA
a(n) = 0 if (n mod 8) not in {0, 1}. - Max Alekseyev, Sep 28 2023
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Fausto A. C. Cariboni, Apr 02 2017
EXTENSIONS
a(28)-a(31) from Max Alekseyev, Sep 24 2023
STATUS
approved