A new proof of a generalization of Gerzon’s bound

https://doi.org/10.1007/s10998-020-00372-9

A new proof of a generalization of Gerzon’s bound

Gábor Hegedüs¹

Accepted: 1 May 2020 / Published online: 22 November 2020

© The Author(s) 2020

Abstract

In this paper we give a short, new proof of a natural generalization of Gerzon’s bound. This bound improves the Delsarte, Goethals and Seidel’s upper bound in a special case. Our proof is a simple application of the linear algebra bound method.

Keywords Gerzon’s bound·Distance problem·Linear algebra bound method Mathematics Subject Classification 52C45·15A03·12D99

1 Introduction

In this paper we give a linear algebraic proof of the known upper bound for the size of some special sphericals-distance sets. This result generalizes Gerzon’s general upper bound for the size of equiangular spherical set.

In the sequel,R[x₁, . . . ,x_n] = R[x]denotes the ring of polynomials in commuting variablesx₁, . . . ,x_noverR.

LetG ⊆Rⁿ be an arbitrary set. Denote byd(G)the set of (non-zero) distances among points ofG:

d(G):= {d(p1,p2): p1,p2∈G,p1=p2}.

Ans-distance set is any subset_H⊆Rⁿsuch that|d(H)| ≤s.

Let(x,y)stand for the standard scalar product. Lets(G)denote the set of scalar products between the distinct points ofG:

s(G):= {(p1,p2): p1,p2∈G,p1=p2}.

Aspherical s-distance set means any subsetG⊆Sⁿ⁻¹such that|s(G)| ≤s.

Letn,s≥1 be integers. Define M(n,s):=

n+s−1 s

+

n+s−2 s−1

.

B

hegedus.gabor@nik.uni-obuda.hu

1 Óbuda University, Bécsi út 96, Budapest 1037, Hungary

Delsarte, Goethals and Seidel investigated the sphericals-distance sets. They proved a general upper bound for the size of a sphericals-distance set in [1].

Theorem 1.1 (Delsarte et al. [1]) Suppose thatF ⊆ Sⁿ⁻¹ is a set satisfying|s(F)| ≤ s.

Then

|F| ≤M(n,s).

Barg and Musin gave an improved upper bound for the size of a sphericals-distance set in a special case in [2]. Their proof builds upon Delsarte’s ideas (see [1]) and they used Gegenbauer polynomials in their argument.

Theorem 1.2 (Barg and Musin [2])Let n≥1be a positive integer and let s>0be an even integer. Let_S ⊆Sⁿ⁻¹denote a spherical s-distance set with inner products t₁, . . . ,t_s such that

t₁+ · · · +t_s≥0. Then

|S| ≤M(n,s−2)+n+2s−2 s

n+s−1 s−1

. We point out the following special case of Theorem1.1.

Corollary 1.3 Suppose thatF⊆Sⁿ⁻¹is a set satisfying|s(F)| ≤2. Then

|S| ≤n(n+3)

2 .

Anequiangular spherical setmeans a two-distance spherical set with scalar productsα and−α. LetM(n)denote the maximum cardinality of an equiangular spherical set. There is a very extensive literature devoted to the determination of the precise value ofM(n)(see [3,4]). Gerzon gave the first general upper bound forM(n)/

Theorem 1.4 (Gerzon [3, Theorem 8])Let n≥1be a positive integer. Then M(n)≤ n(n+1)

2 .

Musin proved a stronger version of Gerzon’s Theorem in [5, Theorem 1]. He used the linear algebra bound method in his proof.

Theorem 1.5 (Musin [5, Theorem 1]) LetS be a spherical two-distance set with inner products a and b. Suppose that a+b≥0. Then

|S| ≤n(n+1)

2 .

De Caen gave a lower bound for the size of an equiangular spherical set.

Theorem 1.6 (de Caen [6])Let t>0be a positive integer. For each n=3·2^2t−1−1there exists an equiangular spherical set of²₉(n+1)²vectors.

Our main result is an alternative proof of a natural generalization of Gerzon’s bound, which improves the Delsarte, Goethals and Seidel’s upper bound in a special case. Our proof uses the linear algebra bound method. The following statement was proved in [7] Theorem 6.1. The original proof builds upon matrix techniques and the addition formula for Jacobi polynomials.

Theorem 1.7 (Delsarte et al. [7, Theorem 6.1]) Let s = 2t > 0be an even number and n > 0be a positive integer. LetS denote a spherical s-distance set with inner products a₁, . . . ,a_t,−a₁, . . . ,−a_t. Suppose that0<a_i <1for each i . Then

|S| ≤

n+s−1 s

.

2 Preliminaries

We prove our main result using the linear algebra bound method and the Determinant Criterion (see [8, Proposition 2.7]). We recall here this principle for the reader’s convenience.

Proposition 2.1 (Determinant Criterion)Let Fbe an arbitrary field. Let f_i : → Fbe functions andv_j ∈elements for each1 ≤i,j ≤m such that the m×m matrix B = (fi(vj))^m_i,j=1is non-singular. Then f1, . . . ,fmare linearly independent.

Consider the set of vectors M(n,s):=

α=(α1, . . . , αn)∈Nⁿ : α1≤1, n

i=1

αi is even, n

i=1

αi≤s

.

Define the set

N(n,s)=

α=(α1, . . . , αn)∈Nⁿ: n i=1

αi≤s

.

Lemma 2.2 Let n,s≥1be integers. Then

|N(n,s)| = n+s

s

.

Proof For a simple proof of this fact see [9, Section 9.2 Lemma 4].

We need the following combinatorial statement.

Lemma 2.3 Let n>0be a positive integer and s>0be an even integer. Then

|M(n,s)| =

n+s−1 s

.

Proof It is easy to check that there exists a bijection f :M(n,s)→N(n−1,s), sinces is even. Namely, letα =(α1, . . . , αn)∈M(n,s)be an arbitrary element. Define f(α):=

(α2, . . . , αn). It is easy to verify that f(M(n,s))⊆N(n−1,s)and f is a bijection.

Hence|M(n,s)| = |N(n−1,s)|and Lemma2.2gives the result.

3 Proof of Theorem1.7

Consider the real polynomial g(x₁, . . . ,x_n)=

_n

m=1

x_m²

−1∈R[x₁, . . . ,x_n].

LetS = {v₁, . . . ,v_r}denote a sphericals-distance set with inner productsa₁, . . . ,a_t,

−a₁, . . . ,−a_t. Herer= |S|. Define the polynomials Pi(x1, . . . ,xn):=

t

m=1

(x,vi)²−(am)²

∈R[x]

for each 1≤i≤r. Clearly deg(P_i)≤s=2tfor each 1≤i≤r.

Consider the set of vectors E(n,s):=

α=(α1, . . . , αn)∈Nⁿ :

i

αi is even, αi ≤s

It is easy to verify that if we can expandPi as a linear combination of monomials, then we get

Pi(x1, . . . ,xn)=

α∈E(n,s)

c_αx^α, (3.1)

wherec_α ∈ Rare real coefficients for eachα ∈ E(n,s) andx^α denotes the monomial x₁^α1·. . .·xn^αn.

Sincevi ∈Sⁿ⁻¹, this means that the equation x₁²=1−

n j=2

x²_j (3.2)

is true for eachvi, where 1≤i≤r. LetQidenote the polynomial obtained by writingPias a linear combination of monomials and replacing, repeatedly, each occurrence ofx₁^t, where t≥2, by a linear combination of other monomials, using the relations (3.2).

Sinceg(v_i)=0 for eachi, henceQ_i(v_j)=P_i(v_j)for each 1≤i =j ≤r.

We prove that the set of polynomials{Qi : 1≤i ≤r}is linearly independent. This fact follows from the Determinant Criterion, when we defineF:=R,=Sⁿ⁻¹and fi := Qi

for each i. It is enough to prove that Qi(vi) = Pi(vi) = 0 for each 1 ≤ i ≤ r and Q_i(v_j)= P_i(v_j)= 0 for each 1≤i = j ≤r, since then we can apply the Determinant Criterion.

ButP_i(v_i) = _m

i=1(1−a²_m)and P_i(v_j) =0, because_S = {v₁, . . . ,v_r}is a spherical s-distance set with inner productsa1, . . . ,at,−a1, . . . ,−at.

It is easy to check that we can writeQias a linear combination of monomials in the form

Qi=

α∈M(n,s)

d_αx^α,

whered_α∈Rare the real coefficients for eachα∈M(n,s). This follows immediately from the expansion (3.1) and from the relation (3.2).

Since the polynomials{Qi : 1≤i ≤r}are linearly independent and if we expandQias a linear combination of monomials, then all monomials appearing in this linear combination contained in the set of monomials

{x^α : α∈M(n,s)}

for eachi, hence

r= |S| ≤ |M(n,s)| =

n+s−1 s

, by Lemma2.3.

4 Concluding remarks

The following Conjecture is a natural generalization of Theorem1.5and a strengthening of Theorem1.2.

Conjecture 4.1 Let n≥1be a positive integer and let s>0be an even integer. LetS⊆Sⁿ⁻¹ denote a spherical s-distance set with inner products t₁, . . . ,t_ssuch that

t₁+. . .+t_s≥0. Then

|S| ≤

n+s−1 s

.

Acknowledgements I am indebted to Lajos Rónyai for his useful remarks.

Funding Open access funding provided by Óbuda University

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