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A351759
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Number of words either empty or beginning with the first letter of the n-ary alphabet, where each letter of the alphabet occurs n times and letters of neighboring word positions are equal or neighbors in the alphabet.
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2
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1, 1, 3, 37, 3593, 2336376, 12085125703, 492300320300251, 163856834420168837331, 450436951076447377275760495, 10360995769563446558192576106611177, 2010994252878923176109086647579398800496256, 3317129856969862808949985510559979253492213624483355
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OFFSET
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0,3
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COMMENTS
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Also the number of (n^2-1)-step walks on n-dimensional cubic lattice from (1,0,...,0) to (n,n,...,n) with positive unit steps in all dimensions such that the absolute difference of the dimension indices used in consecutive steps is <= 1.
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LINKS
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FORMULA
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EXAMPLE
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a(2) = 3:
+----+ +----+ +----+
|aabb| |abab| |abba|
+----+ +----+ +----+
|1222| |1122| |1112|
|0012| |0112| |0122|
+----+ +----+ +----+
|xx | |x x | |x x|
| xx| | x x| | xx |
+----+ +----+ +----+
a(3) = 37: aaabbbccc, aaabbcbcc, aaabbccbc, aaabbcccb, aaabcbbcc, aaabcbcbc, aaabcbccb, aaabccbbc, aaabccbcb, aaabcccbb, aababbccc, aababcbcc, aababccbc, aababcccb, aabbabccc, aabbcccba, aabcbabcc, aabcbccba, aabccbabc, aabccbcba, aabcccbab, aabcccbba, abaabbccc, abaabcbcc, abaabccbc, abaabcccb, abababccc, ababcccba, abbaabccc, abbcccbaa, abcbaabcc, abcbccbaa, abccbaabc, abccbcbaa, abcccbaab, abcccbaba, abcccbbaa.
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MAPLE
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b:= proc(l, t) option remember; (n-> `if`(l=[0$n], 1,
add(`if`(l[i]=0, 0, b(subsop(i=l[i]-1, l), i)),
i=max(1, t-1)..min(n, t+1))))(nops(l))
end:
a:= n-> b([n$n], 0):
seq(a(n), n=0..6);
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MATHEMATICA
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b[l_, t_] := b[l, t] = Function [n, If[l == Array[0&, n], 1,
Sum[If[l[[i]] == 0, 0, b[ReplacePart[l, i -> l[[i]] - 1], i]],
{i, Max[1, t - 1], Min[n, t + 1]}]]][Length[l]];
a[n_] := b[Array[n&, n], 0];
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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