General Bounds on Interleaving Numbers

or Construction 4.2 (depending on which is applicable) to find a zigzag row in H;

thirdly, remove the zigzag row inH to get a [d_|St^A_|+1e(|S_t|+ 1)−1]×l₂ torusT; finally, find non-negative integers xandy such thatl₁ =x(|S_t|+ 1) +y[d_|S^A

t|+1e(|S_t|+ 1)−1], and get an l₁ ×l₂ torus by tiling x copies of B and y copies of T vertically. The resulting interleaving on the l₁×l₂ torus is a t-interleaving.

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requirement on the torus’ size is loosened to some extent (Theorem 3.8), then its t-interleaving number is at most |S_t|+ 2. Does that mean for a torus of any size, its t-interleaving number is always at most |S_t|plus a small constant? The answer is no.

The following theorem shows bounds ont-interleaving numbers.

Theorem 3.12 (1) The t-interleaving numbers of two-dimensional tori are |S_t|+ O(t²) in general. And that upper bound is tight, even if the following restriction is enforced on the tori — the number of rows or the number of columns of the torus approaches infinity. (2) When both l₁ and l₂ are of the order Ω(t²), the t-interleaving number of an l₁×l₂ torus is |S_t|+O(t).

Proof: (1) Firstly, let’s show that the t-interleaving numbers of two-dimensional tori are |S_t|+O(t²) in general. Let G be an l₁ × l₂ torus. First we assume that t is even and l₁ ≥ t, l₂ ≥ t. Let K₁ = b^l_t¹c, K₂ = b^l_t²c. We see G as being tiled by small blocks in the way shown in Fig. 3.12, where the blocks are labelled by ‘A’ or ‘B’. (Note that two blocks both labelled as ‘A’ are not necessary of the same size. And two blocks both labelled as ‘B’ are not necessary of the same size, either.) For every block labelled as ‘A’ (respectively, ‘B’), the four blocks around it (to its left, right, up and down) are all labelled as ‘B’ (respectively, ‘A’). Each block consists of eitherd_2K^l1₁eorb_2K^l1₁crows, and eitherd_2K^l2₂eorb_2K^l2₂ccolumns. (Note that d_2K^l1

1e=d^K1t+(l_2K^1modt)

1 e= ₂^t +d^l1_2K^modt

1 e, b_2K^l1

1c= ₂^t +b^l1_2K^modt

1 c,d_2K^l2

2e= ₂^t +d^l2_2K^modt

2 e, b_2K^l2₂c = ₂^t +b^l2_2K^mod₂ ^tc.) We see each block as a torus of its corresponding size. (So for a block whose size is α×β, it vertices are denoted by (i, j) for i= 0,1,· · · , α−1 and j = 0,1,· · ·, β, in the same way a torus’ vertices are normally denoted.) Now we interleave all the blocks following these two rules: (i) only integers in the set {1,2,· · · ,d_2K^l1₁e · d_2K^l2₂e}are used to interleave any block ‘A’, and only integers in the set {d_2K^l1

1e · d_2K^l2

2e+ 1,d_2K^l1

1e · d_2K^l2

2e+ 2,· · · ,2· d_2K^l1

1e · d_2K^l2

2e} are used to interleave any block ‘B’; (ii) for all the blocks labelled by ‘A’ (respectively, ‘B’) and for any i and j, the vertices denoted by (i, j) in them (provided they exist) are all labelled by the same integer. It is very easy to see that G is t-interleaved in this way, using 2· d_2K^l1

1e · d_2K^l2

2e = 2(₂^t +d^l1_2K^modt

1 e)(₂^t +d^l2_2K^modt

2 e)≤2(₂^t +d^t−1₂ e)(₂^t +d^t−1₂ e) = 2t² =

A B A A B

B A B A B

B

B A A

B

A

A B A A B

B A B

l1

l₂ G

Figure 3.12: See Gas being tiled by small blocks.

|S_t|+ ³₂t² distinct integers. So G’s t-interleaving number is |S_t|+O(t²).

Now we assume t is even, and l₁ < t or l₂ < t. Without loss of generality, let’s say l₁ < t. Then we see G as being tiled horizontally by smaller toriA₁,A₂,· · ·, A_n, where eachA_i— fori= 1,2,· · · , n−1 — is anl₁×ttorus, andA_nis anl₁×(l₂ modt) torus. We interleave A₁, A₂, · · ·, A_n−1 in exactly the same way, and assign l₁ ×t distinct integers to each of them. We interleaveA_nwith a disjoint set ofl₁×(l₂ modt) integers. ClearlyGist-interleaved in this way, usingl₁·t+l₁·(l₂ modt) =|S_t|+O(t²) distinct integers. So again, G’s t-interleaving number is |S_t|+O(t²).

Finally we assumetis odd. We can (t+1)-interleaveGusing|S_t+1|+O((t+1)²) =

(t+1)²

2 +O((t+ 1)²) = ^t2+1₂ +O(t²) =|S_t|+O(t²) distinct integers. t+ 1 is even, and a (t + 1)-interleaving is also a t-interleaving. So G’s t-interleaving number is still

|S_t|+O(t²).

Now let’s show that the above bound on t-interleaving numbers, |S_t|+O(t²), is tight, no matter if t is even or odd. Consider an l₁×l₂ torus where l₁ is the largest even integer that is no greater than b³₂tc, and l₂ is any integer greater than or equal to b³₄tc. We are firstly going to show that a t-interleaving can place an integer at most twice in any b³₄tc consecutive columns of the torus.

Assume a t-interleaving places an integer on three vertices in b³₄tc consecutive columns of the torus. Without loss of generality, let’s say those three nodes are v_0,0, v_i1,j1 and v_i2,j2, where 0 ≤j₁ ≤ b³₄tc −1 and 0≤j₂ ≤ b³₄tc −1. Since the interleaving

is at-interleaving, the Lee distance between any two of those three vertices is at least t. Let a= ^l₂¹ andb =b³₄tc −1. It is not difficult to see that the Lee distance between v_i1,j1 andv_a,bis at most min{(a−i₁) modl₁,(i₁−a) modl₁}+(b−j₁) = ^l₂¹−min{(0−

i₁) mod l₁,(i₁−0) mod l₁}+(b−j₁) = ^l₂¹+b−[min{(0−i₁) mod l₁,(i₁−0) modl₁}+j₁].

Since the Lee distance between v_0,0 and v_i1,j1 is at most min{(0−i₁) modl₁,(i₁ − 0) modl₁}+j₁, we know that min{(0−i₁) mod l₁,(i₁−0) modl₁}+j₁ ≥t. Therefore the Lee distance between v_i1,j1 and v_a,b is at most ^l₂¹ +b−t ≤ b³₂tc/2 +b³₄tc −1−t

< ^t₂. Similarly, the Lee distance between v_i2,j2 and v_a,b is also less than ₂^t. Therefore the Lee distance between v_i1,j1 and v_i2,j2 is less than t, which is a contradiction. So a t-interleaving cannot place an integer on more than two vertices in b³₄tc consecutive columns of the torus.

Anyb³₄tcconsecutive columns of thel₁×l₂torus containl₁×b³₄tc ≥(³₂t−2)×(³₄t−1)

= ⁹₈t²−3t+ 2 vertices, where each integer can be placed on at most two vertices by a t-interleaving. Therefore the t-interleaving number of the torus is at least ^98t2−3t+2₂ =

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16t²−³₂t+ 1 = ^t2₂⁺¹+₁₆¹t²−³₂t+¹₂ ≥ |S_t|+₁₆¹t²−³₂t+¹₂ =|S_t|+ Θ(t²), which matches the upper bound |S_t|+O(t²). Since here l₂ can be any integer that is no less than b³₄tc, the upper bound is tight even if the number of columns (or equivalently, the number of rows) of the torus approaches infinity. The first part of this theorem has been proved by now.

(2) Let’s prove the second part of this theorem. In the previous part of this proof, a method for t-interleaving an l₁×l₂ torus has been proposed for the case ‘t is even and l₁ ≥t,l₂ ≥t’. That method uses 2(₂^t+d^l1_2K^modt

1 e)(₂^t+d^l2_2K^modt

2 e) distinct integers.

(Note that K₁ = b^l_t¹c and K₂ =b^l_t²c.) When both l₁ and l₂ are of the order Ω(t²), bothK1 and K2 are of the order of Ω(t) — and then 2(₂^t+d^l1_2K^mod₁ ^te)(₂^t+d^l2_2K^mod₂ ^te) = 2(₂^t +O(1))(₂^t +O(1)) = ^t₂² +O(t) =|S_t|+O(t). When t is odd, we can t-interleave an l1 ×l2 torus, where l1 = Ω(t²) = Ω((t + 1)²) and l2 = Ω(t²) = Ω((t+ 1)²), by (t+1)-interleaving it using|S_t+1|+O(t+1) = ^(t+1)₂ ²+O(t) = ^t2+1₂ +O(t) = |S_t|+O(t) distinct integers. So no matter if t is even or odd, when both l1 and l2 are of the order Ω(t²), the t-interleaving number of an l₁×l₂ torus is|S_t|+O(t).

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