NOTE ON CYCLIC DECOMPOSITIONS OF COMPLETE BIPARTITE GRAPHS INTO CUBES
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The 1-dimensional cube Q 1 is the graph K 2 while the 2-dimensional cube Q 2 is isomorphic to the cycle C 4 . In general, the d-dimensional hypercube Q d is defined recursively as the product Q d−1 2K 2 . Obviously, such a hypercube has 2 d vertices and d2 d−1 edges. Another nice definition of a hypercube Q d (often called just a cube) can be stated as follows: Take all binary numbers of length d and assign them to the vertices v 1 , v 2 , . . . , v 2d
It was proved by Vanden Eynden that for a given d ≥ 2, the graph K d2d−2
can be cyclically factorized into copies of Q d . In this note we improve Vanden Eynden’s re- sult and show that for a given d ≥ 2 the graph K d2d−2
For d = 4 we want to decompose K d2d−2
Lemma 1. Let d ≥ 3 and let N j d : {1, 2, . . . , 2 d−1 } → Z d2d−2
First we prove that h 1 (d) < (d + 1)2 d−3 . It is indeed true for d = 3. We suppose that h 1 (d − 1) < d2 d−4 for every d ≤ d 0 , and want to show that then it follows that h 1 (d 0 ) < (d 0 + 1)2 d0
Now we can prove that N 1 d and N 2 d are one-to-one. It is obviously true for N 1 3 and N 2 3 . Then we suppose that N 1 d is one-to-one for any d ≤ d 0 and want to show that then it holds that N 1 d0
j ≤ 2 d0
P roof. We suppose that d > 2, as the case d = 2 is trivial. Let r = −2 and s = 2. Then r|m and s|n, as m = n = d2 d−2 and t = gcd(r, s) = 2. It holds that |E(Q d )| = d2 d−1 = gcd(ms, nr). It follows that R = r/t = −1, S = s/t = 1 and k = gcd(Sm, Rn) = d2 d−2 . The function ψ : Z d2d−2
We define the functions N 1 d and N 2 d from V 1 d and V 2 d both into Z d2d−2
that we have labeled the vertices v 1 , v 2 , . . . , v 2d−1
We now define the functions N 1 d+1 , N 2 d+1 and θ d+1 as follows. Sim- ilarly as before, θ d+1 (v i , u j ) = (N 1 d+1 (v i ) + N 2 d+1 (u j ), N 1 d+1 (v i )). For v 1 , v 2 , . . . , v 2d−1
For v 2d−1
θ d+1 (v 2d−1
= (N 1 d+1 (v 2d−1
The remaining labels are assigned to the edges joining the copies of Q d
Therefore this part of the diagonal repeats the structure of the secondary diagonal of the left upper subarray (and hence the structure of the sec- ondary diagonal of Q d ) and contains the labels ((d − 1)2 d−3 + (d + 1)2 d−3 , 0), ((d − 1)2 d−3 + (d + 1)2 d−3 + 1, 0), . . . , ((d − 1)2 d−3 + (d + 1)2 d−3 + 2 d−2 − 1), 0) or, more conveniently, (d2 d−2 , 0), (d2 d−2 + 1, 0), . . . , ((d + 1)2 d−2 − 1, 0), and (d2 d−2 + 1, 1), (d2 d−2 + 2, 1), . . . , ((d + 1)2 d−2 , 1). Similarly, the right up- per part of the secondary diagonal contains the labels ((d + 1)2 d−2 , 0), ((d + 1)2 d−2 + 1, 0), . . . , ((d + 2)2 d−2 − 1, 0) and ((d + 1)2 d−2 + 1, 1), ((d + 1)2 d−2 + 2, 1), . . . , ((d + 2)2 d−2 , 1). This is so because for every pair (v i , u 2d−1
Hence we have checked that the function θ d+1 : E(Q d+1 ) → Z (d+1)2d−1
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