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A329910
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Number of harmoniously labeled graphs with n edges and at most n vertices.
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2
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0, 0, 1, 4, 32, 72, 2187, 20736, 262144, 3200000, 48828125, 729000000, 13060694016, 230539333248, 4747561509943, 96717311574016, 2251799813685250, 51998697814229000, 1350851717672990000, 34867844010000000000, 1000000000000000000000, 28531167061100000000000
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OFFSET
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1,4
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COMMENTS
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A graph G with n edges is harmonious if there is an injection f from its vertex set to the group of integers modulo n such that when each edge uv of G is assigned the weight f(u)+f(v) (mod n), the resulting weights are distinct.
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LINKS
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FORMULA
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For n odd, a(n) = ((n-1)/2)^n. For n even, a(n) = (n*(n-2)/4)^(n/2).
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EXAMPLE
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a(3)=1 because there is only one harmonious graph with 3 edges and at most 3 vertices.
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MATHEMATICA
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Table[If[EvenQ[n], (n*(n-2)/4)^(n/2), ((n-1)/2)^n], {n, 1, 22}] (* Stefano Spezia, Nov 24 2019 *)
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CROSSREFS
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A085526 contains the odd-indexed terms.
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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