Firstly, the defining set of the full (7, 3, 5) design given in the proof as S = {{1, 3, 6}, {1, 4, 5}, {1, 4, 6}, {1, 5, 6}, {2, 3, 4},
}{2, 3, 5}, {2, 3, 6}, {2, 3, 7}, {2, 4, 5}, {2, 4, 6}, {{2, 4, 7}, {2, 5, 7}, {3, 4, 6}, {3, 5, 6}, {4, 5, 6}}
does not intersect the Pasch trade
T ={{1, 2, 3}, {2, 5, 6}, {1, 6, 7}, {3, 5, 7}}
and
T′ ={{1, 7, 3}, {7, 5, 6}, {1, 6, 2}, {3, 5, 2}}.
So by Lemma 3.2, S cannot be a defining set. An alternative proof to Theorem 6 of [2] is given below.
Theorem 3.10 d3(7)≤ 15.
Proof. Changing the block {1,4,5} of S to {2,5,6} yields a correct defining set as we now show.
Let
V ={1, 2, ..., 7}
and
S = {{1, 3, 6}, {1, 4, 6}, {1, 5, 6}, {2, 3, 4}, {2, 3, 5}, }{2, 3, 6}, {2, 3, 7}, {2, 4, 5}, {2, 4, 6}, {2, 4, 7}, {{2, 5, 6}, {2, 5, 7}, {3, 4, 6}, {3, 5, 6}, {4, 5, 6}}.
To complete the proof we simply need to show that S is a defining set of the full (7, 3, 5) design, D = (V, B). We start by assuming that S can be extended to a quasi-full design D′ = (V, B′) and then we show that D′ = D.
Let S′ = B′−S, s′i be the number of blocks of S′ containing the element i, and s′i,j be the number of blocks of S′ containing the pair i, j. We observe that:
• s′6 = 6, s′6,7 = 5, s′1,6 = s′2,6 = 2 so {1, 6, 7}, {2, 6, 7} ∈ S′.
• s′2 = 6, s′1,2 = 5, s′2,3 = s′2,4 = s′2,5 = 1, s′2,6 = s′2,7 = 2 so the five remaining blocks containing 2 also contain 1 and thus
{1, 2, 3}, {1, 2, 4}, {1, 2, 5}, {1, 2, 6}, {1, 2, 7} ∈ S′.
• s′3,6 = s′4,6 = s′5,6 = 1 so the three remaining blocks of S′ containing 6 are {3, 6, 7}, {4, 6, 7}, {5, 6, 7}.
Consider
S′′ = S′\ {{1, 6, 7}, {2, 6, 7}, {1, 2, 3}, {1, 2, 4}, {1, 2, 5}, {1, 2, 6}, S′\ {{1, 2, 7}, {3, 6, 7}, {4, 6, 7}, {5, 6, 7}}.
For v = {2, 6}, s′′v = 0 so only five elements appear in the blocks of S′′. Also B\ P3(V )⊆ S′′ where P3(V ) is the set of all the 3-subsets of set V . Hence the blocks of T = (B\ P3(V ), P3(V )\ B) are based on at most five elements. But since every trade must be based on at least six elements, B \ P3(V ) = ∅ and
thus D = D′.
Critical sets of (m, n, 2)-balanced Latin rectangles
In this chapter we turn our focus to critical sets of (m, n, t)-balanced Latin rectangles. Any (m, n, t)-balanced Latin rectangle R used in this chapter may be represented in two ways. Firstly, as an m × n array of multisets, with the set in cell Rr,c containing λ occurrences of element s if and only if the triple (i, j, s) has multiplicity λ in R (see the Introduction). We may also represent an (m, n, t)-balanced Latin rectangle as an edge-coloured bipartite graph. Given an (m, n, t)-balanced Latin rectangle R, such a graph BR has partite sets N (m) and N (n), with λ edges of colour s between vertices r and c whenever the triple (r, c, s) has multiplicity λ in R. Figure 4.1 gives the two representations of the following (2, 3, 3)-balanced Latin rectangle.
R = {(1, 1, 1), (1, 1, 1), (1, 1, 2), (1, 2, 1), (1, 2, 2), (1, 2, 3), }(1, 3, 2), (1, 3, 3), (1, 3, 3), (2, 1, 2), (2, 1, 3), (2, 1, 3), {(2, 2, 1), (2, 2, 2), (2, 2, 3)(2, 3, 1), (2, 3, 1), (2, 3, 2)}.
We will switch freely between these equivalent representations in this chapter, using whichever form makes proofs easier to follow.
As (n, n, n)-balanced Latin rectangles are n-Latin squares of order n, our aim is to study the structure of the critical sets of the full (m, n, 2)-balanced
Figure 4.1: A (2, 3, 3)-balanced Latin rectangle
Latin rectangle and apply these results to the full n-Latin square in Chapter 5.
In this chapter, we focus on the case t = 2; F is always the full (m, n, 2)-balanced Latin rectangle and a saturated partial (m, n, 2)-2)-balanced Latin rect-angle is one where each non-empty cell contains the entries {1, 2}. The next result, shown in [23], is analogous to Theorem 1.3.
Here we define A(R, S) (where R and S are integral vectors of orders m and n respectively) as a set of all m× n 2-Latin rectangles with entries either 1 or 2, and with row and column sums prescribed by R and S.
Theorem 4.1 Let C be a defining set of F and let A ∈ A(R, S) be an (m, n, 2)-balanced Latin rectangle with |R| = m and |S| = n. Then A ∩ C is a defining set for A.
Proof. Suppose that A ∩ C is not a defining set for A. Then there exists an (m, n, 2)-balanced Latin rectangle A′ ∈ A(R, S) such that A′ ̸= A and A∩ C ⊂ A′. Let T = A\ A′ and T′ = A′\ A. Since A ∩ C ⊂ A ∩ A′, T∩ C = ∅.
Now T and T′ are partial (m, n, 2)-balanced Latin rectangles with the same set of occupied cells, with 1 and 2 occurring the same number of times in each row and column. Thus G := (F \ T ) ⊎ T′ is an (m, n, 2)-balanced Latin rectangle. Furthermore since T ∩ C = ∅, C ⊂ G. Since G ̸= F , C is not a
defining set for F , a contradiction.
In [23], the symbol 2 of an element of A(R, S) is replaced by 0 to give a (0, 1)-matrix. Thus the results in this chapter also give us useful information in identifying defining sets in (0, 1)-matrices.
In Section 4.1, we give examples of critical sets of full (n, n, 2)-balanced Latin rectangles of small orders highlighting the similarities in their struc-tures. We then generalize these examples to give the precise structure of a critical set of the full (m, n, 2)-balanced Latin rectangle in Section 4.2. We do this by describing sets of partial m× n arrays A[a, b] where a and b are vectors of non-negative integers satisfying certain properties. Then in Theo-rem 4.2 and TheoTheo-rem 4.7, we show that if (a, b) is a pair of “good” vectors (see Definition 4.2), then a partial (m, n, 2)-balanced Latin rectangle is a critical set of the full (m, n, 2)-balanced Latin rectangle if and only if it is an element of A[a, b]. This gives a classification of such critical sets. Section 4.3 gives the analogous result (on (m, n, 2)-balanced Latin rectangles) to Lemma 1.2 and we use this result in Section 4.4 to prove Theorem 4.2. In Section 4.5 we show that when m, n ≥ 2, the size of the smallest critical set of the full (m, n, 2)-balanced rectangle is (m− 1)(n − 1) + 1 and the size of the largest critical set of the full (m, n, 2)-balanced Latin rectangle is 2(m− 1)(n − 1).
To conclude this chapter we generalize the definition of A[a, b] for the full (m, n, 2)-balanced Latin rectangle (in Section 4.2) to construct critical sets of the (m, n, t)-balanced Latin rectangle.
Section 4.2 to Section 4.5 is joint work with Nicholas Cavenagh and is published in [23].
4.1 Examples of critical sets of the full (n, n, 2)-balanced Latin rectangle
In this section we give examples of critical sets of the full (n, n, 2)-balanced rectangle for 2≤ n ≤ 4. We then describe some patterns which we generalize formally in the next section.
1,2 1,2 1,2 2
For n = 4, eight of the critical sets are saturated and some of the non-saturated critical sets are given below.
1,2 1,2 1,2
1,2 1,2
We make the following observations. For n = 3, 4, a saturated critical set contains the most entries in each case and the last critical sets for both values of n contain the smallest number of entries with sizes 2(n−1)2 and (n−1)2+ 1
respectively. For n = 5,
1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2
and
2
1 1 1 1
1 1 1 1
1 1 1 1
1 1 1 1
are also the largest and smallest critical sets of the full (5, 5, 2)-balanced Latin square. We can generalize this observation as follows:
1. The partial (n, n, 2)-Latin square
1, 2 1, 2 · · · 1, 2 1, 2 1, 2 · · · 1, 2
... ... . .. ... ... 1, 2 1, 2 · · · 1, 2
· · ·
of order n and size 2(n− 1)2 is a critical set of the full (n, n, 2)-balanced Latin square.
2. The partial (n, n, 2)-Latin square
· · · 2 1 1 · · · 1 1 1 · · · 1
... ... . .. ... ...
1 1 · · · 1
of order n and size (n−1)2+1 is a critical set of the full (n, n, 2)-balanced Latin square.
We will later prove that these are the largest and smallest critical sets, respec-tively, in Section 4.5.
Observe that each critical set example presented so far in this chapter may be described as in Figure 4.2 where each grey block is a saturated critical set of the corresponding full 2-balanced Latin rectangle of the same size. We make use of this observation in the next two sections to fully describe the critical sets of the full (m, n, 2)-balanced rectangle (see [23]).
2
. ..
1
Figure 4.2: Critical set of the full (m, n, 2)-balanced Latin rectangle