347 Introduction to Delsarte Theorycombin/ac2014/docs/tanaka.pdf · Introduction to Delsarte Theory...
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Delsarte理論入門Introduction to Delsarte Theory
Hajime Tanaka
Research Center for Pure and Applied MathematicsGraduate School of Information Sciences
Tohoku University
June 16, 2014Algebraic Combinatorics Summer School 2014
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What is Delsarte Theory?
Philippe Delsarte,An algebraic approach to the association schemes of codingtheory,Philips Res. Rep. Suppl. No. 10 (1973).
... studies codes and designs within the unifying framework ofassociation schemes;... has been playing a central role in Algebraic Combinatorics;... is still important. Applications include extremal set theory andfinite geometry.
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What is Delsarte Theory?
Keywords:
codesdesignslinear programming boundP-polynomial and Q-polynomial properties4 fundamental parameters (including minimum distance andstrength)duality (in the case of translation association schemes)
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Coding theory (in a very general form)
X : a finite set = a set of codewords
X channel receiver
noise
x ∼ y def⇐⇒ x and y can be “confused” (x, y ∈ X)
Find a large subset C of X which can be sent without confusion !!
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Coding theory: an example
0
1
2
3
4
5
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Coding theory: an example
C = 0
0
1
2
3
4
5
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Coding theory: an example
C = 0, 1
0
1
2
3
4
5
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Coding theory: an example
C = 0, 2
0
1
2
3
4
5
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Coding theory: an example
C = 0, 2, 40
1
2
3
4
5
Find the independence number of the graph !!a good upper bound on
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Coding theory: another example
H(6, 2) = (X, Ri6i=0) : the binary Hamming scheme of class 6
X = 0, 16
Consider the binary symmetric channel with error probability p≪ 1
0
1
0
1
1 − p
1 − p
p p
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Coding theory: another example
ExampleLet us send 000000.Prob[ 000000 7−→ 100000 ] = p(1− p)5
Prob[ 1 error occurs ] = 6× p(1− p)5
Prob[ 000000 7−→ 110000 ] = p2(1− p)4
Prob[ 2 errors occur ] =(6
2
)× p2(1− p)4 = 15× p2(1− p)4
Prob[ 3 errors occur ] =(6
3
)× p3(1− p)3 = 20× p3(1− p)3
20× p3(1− p)3 ≪ 15× p2(1− p)4 ≪ 6× p(1− p)5
Ignore this possibility, i.e., at most 2 errors occur !!
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Coding theory: another example
We assume that at most 2 errors occur.
Example000000 and 100000 can be confused.000000 and 110000 can be confused.000000 and 111000 can be confused. Indeed:
000000 7−→ 100000 ←−[ 111000
000000 and 111100 can still be confused. Indeed:
000000 7−→ 110000 ←−[ 111100
000000 and 111110 (or 111111) can not be confused.
x ∼ y ⇐⇒ 0 < ∂(x, y) < 5 the Hamming distance
The edge set of our graph is R1 ∪ R2 ∪ R3 ∪ R4.
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Notation
X = (X, Ridi=0) : a symmetric association scheme
A0,A1, . . . ,Ad : the adjacency matrices
ATi = Ai
A = ⟨A0,A1, . . . ,Ad⟩ : the Bose–Mesner algebra (over R)E0,E1, . . . ,Ed : the primitive idempotents
Ei = ETi = Ei
P,Q : the first and second eigenmatrices, i.e.,
Ai =
d∑j=0
Pj,i Ej, Ei =1|X|
d∑j=0
Qj,i Aj
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M-codes
M ⊆ 1, 2, . . . , dC ⊆ X : an M-code
def⇐⇒ C : an independent set of the graph(X,∪
i∈M Ri)
⇐⇒ (C × C) ∩∪
i∈M Ri = ∅⇐⇒ (C × C) ∩ Ri = ∅ for ∀i ∈ M
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A review
B : a (real) n× n matrixu ∈ Rn : an n-dimensional column vector
uTB u =
n∑i,j=1
Bi,j uiuj
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M-codes (continued)
C ⊆ X
χ ∈ RX : the (column) characteristic vector of C, i.e.,
χx =
1 if x ∈ C0 if x ∈ C
(x ∈ X)
χTAiχ =∑
x,y∈X
(Ai)x,y χxχy =∣∣(C × C) ∩ Ri
∣∣C ⊆ X : an M-code⇐⇒ (C × C) ∩ Ri = ∅ for ∀i ∈ M
⇐⇒ χTAiχ = 0 for ∀i ∈ M
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t-(v, d, λ) designs
Ω : a finite set with |Ω| = v(Ωk
): the set of k-subsets of Ω
D ⊆(Ωd
): a t-(v, d, λ) design
def⇐⇒ ∀z ∈(Ωt
) ∣∣x ∈ D : z ⊆ x∣∣ = λ
Example1
2
3 4 5
67
a 2-(7, 3, 1) design
Given t, v, d, find a small t-(v, d, λ) design !!
(vt
)λ =
(dt
)|D|
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t-(v, d, λ) designs
J(v, d) = (X, Ridi=0) : the Johnson scheme
X =(Ωd
)D ⊆ X
χ ∈ RX : the characteristic vector of D
Theorem (Delsarte, 1973)D : a t-(v, d, λ) design (for some λ)⇐⇒ χTEiχ = 0 for ∀i ∈ 1, 2, . . . , t
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M-designs
More generally:
M ⊆ 1, 2, . . . , d
D ⊆ X : an M-design def⇐⇒ χTEiχ = 0 for ∀i ∈ M
RemarkC ⊆ X : an M-code ⇐⇒ χTAiχ = 0 for ∀i ∈ M
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M-designs
Theorem (Delsarte, 1973)Suppose X = H(d, q).D : a 1, 2, . . . , t-design⇐⇒ D : an orthogonal array of strength t
RemarkMany more concepts of t-designs can be viewed as DelsarteM-designs in some association schemes.See, e.g., [7, Section 8].
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A review
B : a real symmetric n× n matrixB : positive semidefinite
def⇐⇒ uTB u =
n∑i,j=1
Bi,j uiuj ⩾ 0 for ∀u ∈ Rn
η1, η2, . . . , ηn ∈ R : the eigenvalues of BB : positive semidefinite ⇐⇒ η1 ⩾ 0, η2 ⩾ 0, . . . , ηn ⩾ 0
Proof.∃U : an orthogonal matrix (i.e., U−1 = UT) s.t.
UTB U = Diag (η1, η2, . . . , ηn)
Set v = UTu = U−1u.
uTB u = (Uv)TB (Uv) = vT(UTB U)v =
n∑i=1
ηi v2i
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The linear programming bound for M-codes
C ⊆ X : an M-codeχ ∈ RX : the characteristic vector of C
χTAiχ =∑
x,y∈X
(Ai)x,y χxχy =∣∣(C × C) ∩ Ri
∣∣ ⩾ 0
i = 0 =⇒ χTA0χ = |C| (∵ A0 = I)i ∈ M =⇒ χTAiχ = 0
χTJ χ =∑
x,y∈X
Jx,y χxχy = |C × C| = |C|2
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The linear programming bound for M-codes
Ei : a real symmetric matrixE2
i = Ei ⇐⇒ Ei(Ei − I) = 0⇐⇒ every eigenvalue is 0 or 1=⇒ Ei : positive semidefinite
χTEi χ ⩾ 0
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The linear programming bound for M-codes
χTA0χ χTAiχ χTAiχ χTJ χ χTEi χi ∈ M i ∈ 1, 2, . . . , d\M i ∈ 1, 2, . . . , d
|C| 0 ⩾ 0 |C|2 ⩾ 0
Set ei :=χTAiχ
|C|. e = (e0, e1, . . . , ed) : the inner distribution of C
χTJ χ = χT
(d∑
i=0
Ai
)χ = |C|
d∑i=0
ei
χTEi χ = χT
1|X|
d∑j=0
Qj,i Aj
χ =|C||X|
d∑j=0
Qj,i ej
e0 ei ei∑d
i=0 ei∑d
j=0 Qj,i ej
i ∈ M i ∈ 1, 2, . . . , d\M i ∈ 1, 2, . . . , d
1 0 ⩾ 0 |C| ⩾ 0
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The linear programming bound for M-codes
View the ei as real variables !!
Theorem (Delsarte, 1973)Consider the following linear programming problem:
maximize ϑ =d∑
i=0
ei
subject to e0 = 1ei = 0 for i ∈ Mei ⩾ 0 for i ∈ 1, 2, . . . , d\M
d∑j=0
Qj,i ej ⩾ 0 for i ∈ 1, 2, . . . , d
If C is an M-code, then |C| ⩽ ϑ.
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The linear programming bound for M-codes
RemarkLinear programming problems can be solved by the simplexmethod.
ExampleSuppose X = H(16, 2), M = 1, 2, 3, 4, 5.Then ϑ = 256.This is attained by the Nordstrom–Robinson code.
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Remarks on the linear programming bound
Delsarte’s linear programming bound, combined with the duality oflinear programming, provides us with the most powerful methodfor bounding the sizes of general M-codes in associationschemes.See, e.g., [6].
Similarly, Delsarte formulated the linear programming (lower)bound for the sizes of M-designs.
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P-polynomial association schemes
Definition (Delsarte, 1973)X : P-polynomial w.r.t. the ordering Rid
i=0def⇐⇒ ∃v0(t), v1(t), . . . , vd(t) ∈ R[t], ∃θ0, θ1, . . . , θd ∈ R s.t.
deg vi(t) = i (0 ⩽ i ⩽ d)
Pj,i = vi(θj) (0 ⩽ i, j ⩽ d)
RemarkBy the orthogonality relation of P, the vi(t) form a system oforthogonal polynomials:
d∑ℓ=0
vi(θℓ) vj(θℓ)mℓ = δi, j · |X| ki (0 ⩽ i, j ⩽ d)
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P-polynomial association schemes
ExampleThe Hamming scheme H(d, q)
The Johnson scheme J(v, d)
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Distance-regular graphs
Γ = (X,R) : a distance-regular graph with diameter d
x
y
ciai
bi
Γ i (x)Γ i−1(x) Γ i+1(x)
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Distance-regular graphs
x
y
ciai
bi
Γ i (x)Γ i−1(x) Γ i+1(x)∂ : the path-length distanceAi : the i th distance matrix:
(Ai)x,y =
1 if ∂(x, y) = i0 otherwise
A0 = I
A0 + A1 + · · ·+ Ad = J
A1Ai = bi−1 Ai−1 + ai Ai + ci+1 Ai+1 where A−1 = Ad+1 = 0
(A1Ai)y,x =∣∣Γ1(y) ∩ Γi(x)
∣∣
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Distance-regular graphs =⇒ P-polynomial schemes
A0 = I
A1Ai = bi−1 Ai−1 + ai Ai + ci+1 Ai+1 where A−1 = Ad+1 = 0A0 + A1 + · · ·+ Ad = J
X = (X, Ridi=0) : a symmetric association scheme, where
Ri := (x, y) : ∂(x, y) = i (0 ⩽ i ⩽ d)
Set v0(t) = 1, v1(t) = t, and
t vi(t) = bi−1 vi−1(x) + ai vi(x) + ci+1 vi+1(t) (1 ⩽ i ⩽ d − 1).
Ai = vi(A1) =⇒ Pj,i = vi(θj) where θj := p1(j)
X : P-polynomial w.r.t. the ordering Ridi=0
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P-polynomial schemes =⇒ distance-regular graphs
RemarkConversely, if a symmetric association scheme X is P-polynomialw.r.t. the ordering Rid
i=0 then the graph Γ = (X,R1) isdistance-regular.This follows from the three-term recurrence relation for a systemof orthogonal polynomials.
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Codes in P-polynomial schemes
Suppose X is P-polynomial w.r.t. the ordering Ridi=0.
C ⊆ X ( 1 < |C| < |X| )χ ∈ RX : the characteristic vector of C
δ := mini = 0 : χTAiχ = 0 : the minimum distance of C
= max
i = 0 : C is a 1, 2, . . . , i− 1-code
s∗ :=∣∣i = 0 : χTEiχ = 0
∣∣ : the dual degree of C
????
Theorem (Delsarte, 1973)δ ⩽ 2s∗ + 1;If δ ⩾ 2s∗ − 1 then C is completely regular.
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Codes in P-polynomial schemes
Complete regularity of C is illustrated as follows:
C y
γ i
α i
βi
Γ i (C)Γ i−1(C) Γ i+1(C)
ρ := max∂(x,C) : x ∈ X : the covering radius of C
χi ∈ RX : the characteristic vector of Γi(C) (0 ⩽ i ⩽ ρ)
A1χi = βi−1 χi−1 + αi χi + γi+1 χi+1 where χ−1 = χρ+1 = 0
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The role of s∗: the outer distribution of C
s∗ =∣∣i = 0 : χTEiχ = 0
∣∣ computable from the inner distribution
Remark
χTE0χ =χTJ χ|X|
> 0
χTEiχ = χT(Ei)2χ = (Eiχ)
T(Eiχ) = ||Eiχ||2, so that
s∗ =∣∣i = 0 : Eiχ = 0
∣∣.
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The role of s∗: the outer distribution of C
B = [A0χ, A1χ, . . . , Adχ ] : the outer distribution of C:
Bx,i = (Aiχ)x =∑y∈X
(Ai)x,y χy =∣∣Γi(x) ∩ C
∣∣ (x ∈ X, 0 ⩽ i ⩽ d)
Recall Eiχ =1|X|
d∑j=0
Qj,i Ajχ.
1|X|
BQ = [E0χ, E1χ, . . . , Edχ ]
rank B = rank BQ = s∗ + 1
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The role of s∗: the outer distribution of C
As an example, suppose X = H(4, 2) and C = 0000, 1111:
0000
0100
0010
00010101
0011
0110
0111
1000
1100
1010
1001
1101
1011
1110
1111
covering radius
δ = 4, ρ = 2, s∗ = 2 =⇒ δ ⩾ 2s∗ − 1minimum distance dual degree
Some of the rows of B: 1 0 0 0 10 1 0 1 00 0 2 0 0
0000th row
0001th row
0110th rowBx,i =
∣∣Γi(x) ∩ C∣∣
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The role of s∗: the outer distribution of C
Some of the rows of B:1 0 0 0 1
0 1 0 1 00 0 2 0 0
0000th row
0001th row
0110th row
ρth column
ObservationA0χ,A1χ, . . . ,Aρχ : linearly independent
Theorem (Delsarte, 1973)ρ ⩽ s∗
hard to compute in general
Proof.dim⟨A0χ,A1χ, . . . ,Adχ⟩ = rank B = s∗ + 1
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Perfect codes and Lloyd polynomials
Theorem (Delsarte, 1973)δ ⩽ 2s∗ + 1;If δ ⩾ 2s∗ − 1 then C is completely regular.
Ur(x) := y ∈ X : ∂(x, y) ⩽ r : the “ball” of radius r centered at x
C : perfect def⇐⇒ X =⨿x∈C
Uρ(x)
L r(t) := v0(t) + v1(t) + · · ·+ vr(t) : the Lloyd polynomial of degree r
Theorem (Delsarte, 1973; “Lloyd Theorem”)δ = 2s∗ + 1 ⇐⇒ C : perfect =⇒ ρ = s∗ and Lρ(t) has ρ simple
roots in θ0, θ1, . . . , θd.
an extremely strong condition !!
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Perfect codes and Lloyd polynomials
Theorem (Delsarte, 1973; “Lloyd Theorem”)δ = 2s∗ + 1 ⇐⇒ C : perfect =⇒ ρ = s∗ and Lρ(t) has ρ simple
roots in θ0, θ1, . . . , θd.
Example
000
100
010
001101
011
110
111=U1(000)
=U1(111)
X = H(3, 2)
C = 000, 111L1(t) = 1 + t
v0(t) v1(t)
θ0, θ1, θ2, θ3 = 3, 1,−1,−3
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Q-polynomial association schemes
Definition (Delsarte, 1973)X : Q-polynomial w.r.t. the ordering Eid
i=0def⇐⇒ ∃v∗0(t), v∗1(t), . . . , v∗d(t) ∈ R[t], ∃θ∗0 , θ∗1 , . . . , θ∗d ∈ R s.t.
deg v∗i (t) = i (0 ⩽ i ⩽ d)
Qj,i = v∗i (θ∗j ) (0 ⩽ i, j ⩽ d)
RemarkBy the orthogonality relation of Q, the v∗i (t) form a system oforthogonal polynomials:
d∑ℓ=0
v∗i (θ∗ℓ ) v∗j (θ
∗ℓ ) kℓ = δi, j · |X|mi (0 ⩽ i, j ⩽ d)
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Q-polynomial association schemes
ExampleThe Hamming scheme H(d, q)
The Johnson scheme J(v, d)
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Designs in Q-polynomial schemes
Suppose X is Q-polynomial w.r.t. the ordering Eidi=0.
D ⊆ X ( 1 < |D| < |X| )χ ∈ RX : the characteristic vector of D
τ := mini = 0 : χTEiχ = 0 − 1 : the (maximum) strength of D
= max
i : D is a 1, 2, . . . , i-design
s :=∣∣i = 0 : χTAiχ = 0
∣∣ : the degree of D
Theorem (Delsarte, 1973)τ ⩽ 2s;If τ ⩾ 2s− 2 then
(D,(D× D) ∩ Ri
di=0
)is a Q-polynomial
scheme (called a subscheme).
remove empty relations
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Tight designs and Wilson polynomials
Theorem (Delsarte, 1973)τ ⩽ 2s;If τ ⩾ 2s− 2 then
(D,(D× D) ∩ Ri
di=0
)is a Q-polynomial
subscheme.
In general, |D| ⩾ m0 + m1 + · · ·+ m⌊τ/2⌋ (the Fisher type bound).
D : tight def⇐⇒ |D| = m0 + m1 + · · ·+ m⌊τ/2⌋
Wr(t) := v∗0(t) + v∗1(t) + · · ·+ v∗r (t) : the Wilson polynomial ofdegree r
Theorem (Delsarte, 1973)τ = 2s ⇐⇒ D : tight =⇒ Ws(t) has s simple roots
in θ∗0 , θ∗1 , . . . , θ∗d.
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Translation association schemes
Suppose X = (X, Ridi=0) is a translation scheme on the abelian
group X.X = (X, Sid
i=0) : the dual of X, where X : the character group of X
Remark
X : P-polynomial ⇐⇒ X : Q-polynomialX : Q-polynomial ⇐⇒ X : P-polynomial
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Translation association schemes
C ⩽ X : a subgroup of X
C⊥ := f ∈ X : f (x) = 1 for ∀x ∈ C ⩽ X
Theorem (Delsarte, 1973)Let M ⊆ 1, 2, . . . , d.C : an M-code ⇐⇒ C⊥ : an M-designC : an M-design ⇐⇒ C⊥ : an M-code
In particular, if X is P-polynomial and / or Q-polynomial, then
δ(C) = τ(C⊥) + 1, s∗(C) = s(C⊥),
τ(C) = δ(C⊥)− 1, s(C) = s∗(C⊥).
minimum distancestrength
dual degreedegree
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Further reading
Classification problem of P- & Q-polynomial schemes
See, e.g., [1, Chapter III], [2, Chapters 8,9], [4, Section 5].The Terwilliger algebra; cf. [9]Orthogonal polynomials, Leonard pairs, tridiagonal pairs; cf. [10]
Two more fundamental parameters (width w, dual width w∗) [3]
The semidefinite programming bound [8]
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References I
[1] E. Bannai and T. Ito, Algebraic combinatorics I: Associationschemes, Benjamin/Cummings, Menlo Park, CA, 1984.
[2] A. E. Brouwer, A. M. Cohen and A. Neumaier, Distance-regulargraphs, Springer-Verlag, Berlin, 1989.
[3] A. E. Brouwer, C. D. Godsil, J. H. Koolen, and W. J. Martin, Widthand dual width of subsets in polynomial association schemes, J.Combin. Theory Ser. A 102 (2003) 255–271.
[4] E. R. van Dam, J. H. Koolen, and H. Tanaka, Distance-regulargraphs, in preparation.
[5] P. Delsarte, An algebraic approach to the association schemes ofcoding theory, Philips Res. Rep. Suppl. No. 10 (1973).
[6] P. Delsarte and V. I. Levenshtein, Association schemes and codingtheory, IEEE Trans. Inform. Theory 44 (1998) 2477–2504.
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References II
[7] W. J. Martin and H. Tanaka, Commutative association schemes,European J. Combin. 30 (2009) 1497–1525; arXiv:0811.2475.
[8] A. Schrijver, New code upper bounds from the Terwilliger algebraand semidefinite programming, IEEE Trans. Inform. Theory 51(2005) 2859–2866.
[9] P. Terwilliger, The subconstituent algebra of an associationscheme I, J. Algebraic Combin. 1 (1992) 363–388.
[10] P. Terwilliger, An algebraic approach to the Askey scheme oforthogonal polynomials, in: F. Marcellan and W. Van Assche(Eds.), Orthogonal polynomials and special functions, Computationand applications, Lecture Notes in Mathematics, vol. 1883,Springer-Verlag, Berlin, 2006, pp. 255–330; arXiv:math/0408390.
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