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A209183
Number of words, either empty or beginning with the first letter of the cyclic 4-ary alphabet, where each letter of the alphabet occurs n times and letters of neighboring word positions are equal or neighbors in the alphabet.
2
1, 2, 62, 2830, 151686, 8893482, 552309938, 35702836038, 2377145582550, 161906392007554, 11227409430866262, 790011772823243214, 56264746328351077194, 4048156319577916177530, 293797748889879887735802, 21483000387938509658756790, 1581177100760460768472276086
OFFSET
0,2
COMMENTS
The first and the last letters are considered neighbors in a cyclic alphabet. The words are not considered cyclic here.
a(n) is also the number of (4*n-1)-step walks on 4-dimensional cubic lattice from (1,0,0,0) to (n,n,n,n) with positive unit steps in all dimensions such that the indices of dimensions used in consecutive steps differ by less than 2 or are in the set {1,4}.
CROSSREFS
Column k=4 of A208879.
Sequence in context: A226421 A091817 A239786 * A200802 A262005 A296278
KEYWORD
nonn,walk
AUTHOR
Alois P. Heinz, Mar 05 2012
STATUS
approved