Original problem 121
Show that there must exist i, j and m, n such that Tᵢⱼ ∩ Tₘₙ ≠ ∅, and i ≠ m or j ≠ n.
Proof of problem 121
By definition, the sets Rᵢⱼ are mutually disjoint, since they are intersections of R with disjoint grid squares Iᵢⱼ, and their union exactly constitutes the region R.
Thus, the sum of their areas equals the area of R:
Σᵢⱼ A(Rᵢⱼ) = A(R).
We are given that A(R) > 1.
The set Tᵢⱼ is created by translating Rᵢⱼ by the integer vector -(i, j). Because translation strictly preserves area, A(Tᵢⱼ) = A(Rᵢⱼ).
Furthermore, the translation shifts every piece into the unit square S = [0, 1) × [0, 1), meaning Tᵢⱼ ⊂ S for all i, j.
Assume for the sake of contradiction that all the translated pieces Tᵢⱼ are mutually disjoint. Then the total area of their union would be the sum of their individual areas, and this union would be entirely contained within S:
Area(⋃ᵢⱼ Tᵢⱼ) = Σᵢⱼ A(Tᵢⱼ) ≤ A(S) = 1.
However, Σᵢⱼ A(Tᵢⱼ) = Σᵢⱼ A(Rᵢⱼ) = A(R) > 1. This creates a contradiction.
Therefore, our assumption is false, and at least two translated sets must overlap. There must exist (i, j) ≠ (m, n) such that Tᵢⱼ ∩ Tₘₙ ≠ ∅.
Original problem 122
Complete the proof of Blichfeldt's Theorem.
Proof of problem 122
From Exercise 121, there exists an overlapping point p ∈ Tᵢⱼ ∩ Tₘₙ with (i, j) ≠ (m, n).
Because p ∈ Tᵢⱼ, it was translated from some point p₁ = (x₁, y₁) ∈ Rᵢⱼ ⊂ R. Thus, p₁ - (i, j) = p.
Because p ∈ Tₘₙ, it was also translated from some point p₂ = (x₂, y₂) ∈ Rₘₙ ⊂ R. Thus, p₂ - (m, n) = p.
Equating these expressions gives p₁ - (i, j) = p₂ - (m, n).
Rearranging gives:
p₁ - p₂ = (i, j) - (m, n) = (i - m, j - n).
Since i, j, m, and n are integers, the difference vector (i - m, j - n) consists entirely of integers. Therefore, x₁ - x₂ ∈ Z and y₁ - y₂ ∈ Z.
Finally, since we chose distinct grid intervals, the points p₁ and p₂ must be distinct. This finds two distinct points in R whose difference is an integer point, completing the proof of Blichfeldt's Theorem.
Original problem 124
Let R be a bounded, convex region in R² that is symmetric about the origin and has area greater than 4. Consider R′ = {(1 / 2)x such that x ∈ R}. Since R′ is a smaller version of R, it is convex and symmetric about the origin. Show that there are points x′ and y′ in R′ such that x′ - y′ is a nonzero lattice point.
Proof of problem 124
Blichfeldt's Theorem states that for any measurable set S ⊂ Rⁿ with volume greater than 1, there exist two distinct points u, v ∈ S such that u - v ∈ Zⁿ \ {0}.
First, calculate the area of R′. Since R′ is defined by scaling the coordinates of R by a factor of 1 / 2, the area scales by the square of that factor:
Area(R′) = (1 / 2)² Area(R) = (1 / 4)Area(R).
Given that Area(R) > 4, we have Area(R′) > (1 / 4)(4) = 1.
Since Area(R′) > 1, Blichfeldt's Theorem gives distinct points x′, y′ ∈ R′ such that x′ - y′ ∈ Z² \ {0}.
This confirms there is a nonzero lattice point formed by the difference of two points in R′.
Original problem 125
Let x′ and y′ be as in Exercise 124. Show that x′ - y′ is in R. Hint: express x′ - y′ as a linear combination of points that you know are in R.
Proof of problem 125
By the definition of R′, since x′, y′ ∈ R′, there exist points x, y ∈ R such that x′ = (1 / 2)x and y′ = (1 / 2)y.
We examine the difference:
x′ - y′ = (1 / 2)x - (1 / 2)y = (1 / 2)x + (1 / 2)(-y).
Since R is symmetric about the origin, if y ∈ R, then -y ∈ R.
Since R is convex, any convex combination of points in R must also be in R.
The expression (1 / 2)x + (1 / 2)(-y) is a convex combination, specifically the midpoint, of x and -y, both of which are in R.
Therefore, x′ - y′ ∈ R. This proves that there exists a nonzero lattice point inside R, which concludes the proof of Minkowski's Theorem.