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Exploring the Set of Square Matrices with Circulant Operators
B. Birregah, Prosper K. Doh, Kondo Adjallah
To cite this version:
B. Birregah, Prosper K. Doh, Kondo Adjallah. Exploring the Set of Square Matrices with Circulant
Operators. WSEAS Transactions on Mathematics, World Scientific and Engineering Academy and
Society (WSEAS), 2007, 6 (2), pp.281-288. �hal-03045126�
Exploring the Set of Square Matrices with Circulant Operators
BABIGA BIRREGAH University of Lom´e College of Science, Applied Math. Dpt. BP. 1515
TOGO
bbirregah@tg.refer.org
PROSPER K. DOH University of Nancy 2 23, boulevard Albert 1er - BP 3397
F54015 Nancy Cedex FRANCE
Prosper.Doh@univ-nancy2.fr
KONDO H. ADJALLAH University of Technology of Troyes
Institute Charles Delaunay 12 rue Marie Curie, F.10010 Troyes Cedex
FRANCE kondo.adjallah@utt.fr Abstract: Circulant operators are useful tools to highlight the way in which the twelve triangular matrix forms of Pascal triangle (collectively referred to as G-matrices) relate to each other. In this paper, we recall the notions of G-matrices and circulant operators, and then we present some main algebraic properties of these operators. We generalize these operators to the case of any square matrix.
Key–Words: Pascal Triangle, Pascal Matrices, k-Circulant Operators, Square Matrix bipartition, cobweb Partition, G-matrices.
1 Introduction
This section recalls the definition of the twelve G- matrices, G k,n 1 ≤ k ≤ 12 as in [2, 22]. In the sequel, we write [G k,n ] ij to denote the coefficient at the intersection of row i and column j and denote by G n the set of the twelve G-matrices. Generic bino- mial coefficients a!
(a − b)!b! will be denoted a b . The Pascal triangle is assumed to comprise levels 0 . . . n.
1.1 The set G n of G-matrices
The following definitions present the twelve triangu- lar matrix forms of the Pascal triangle. They are orga- nized into four subsets.
Definition 1 - The NE/sw G-matrices G 1,n , G 5,n and G 9,n
[G 1,n ] ij =
i
j
if i ≤ j
0 otherwise
(1)
[G 5,n ] ij =
n − i n − j
i, j = 0, ..., n (2)
[G 9,n ] ij =
n + i − j i
if i ≤ j
0 otherwise
(3)
G n N E/sw = {G 1,n , G 5,n , G 9,n }
Definition 2 - The SE/nw G-matrices G 2,n , G 6,n and G 10,n
[G 2,n ] ij =
j n − i
i, j = 0, ..., n (4)
[G 6,n ] ij =
2n − i − j n − j
if i + j ≥ n
0 otherwise
(5)
[G 10,n ] ij =
i n − j
i, j = 0, ..., n (6)
G n SE/nw = {G 2,n , G 6,n , G 10,n }
Definition 3 - The SW/ne G-matrices G 3,n , G 7,n and G 11,n
[G 3,n ] ij =
n + j − i j
if i ≥ j
0 otherwise
(7)
[G 7,n ] ij =
n − j n − i
i, j = 0, ..., n (8) [G 11,n ] ij =
i j
i, j = 0, ..., n (9)
G n SW/ne = {G 3,n , G 7,n , G 11,n }
Definition 4 - The NW/se G-matrices G 4,n , G 8,n and G 12,n
[G 4,n ] ij =
n − i j
i, j = 0, ..., n (10) [G 8,n ] ij =
n − j i
i, j = 0, ..., n (11)
[G 12,n ] ij =
i + j i
if i + j ≤ n
0 otherwise
(12) G n N W/se = {G 4,n , G 8,n , G 12,n }
Notation:
G n = G n N W/sw ∪ G n SE/nw ∪ G n SW/ne ∪ G n N W/se (13) It is easy to observes that:
Theorem 5 If B 6= B ′ and b 6= b ′ then G n B/b ∩ G n B
′/b
′= ∅
Proof:
Considering definitions 1-4, one can easily establish this statement.
Matrices like the above and other matrix forms de- rived from the Pascal triangle are encountered in [1, 3, 4, 9, 10, 16, 8]. Every author simply refers to a particular form encountered as the Pascal matrix.
Some authors refer to G 1, n and G 11 ,n respectively as upper and lower triangular Pascal matrices [5, 19, 17, 18, 20, 21]. In [7, 8], the fourth G-matrix, G 4, n is called binomial matrix. But, as it can be seen from the above definitions, this is just one of the three possible upper-left triangular forms. In some works, one rec- ognizes G 12, n , the third northwest triangular matrix form. [12] refers to G 7, n as the reflection of G 11, n about the main anti-diagonal.
Considering the matrix formulation (14) of the well- known binomial theorem as proposed in [3],
1 (1 + t) (1 + t) 2 . . . (1 + t) n
=
1 t t 2 . . . t n
0 0
1
0
. . . . n 0 0 1 1
. . . . n 1 .. . . .. ... . . . .. . .. . . .. ... .. . 0 . . . . . . 0 n n
| {z }
G
1, n(14)
these matrices can be seen as some reordering of the components of the polynomial power basis vector, and hence of the polynomial space [3].
To our knowledge, this is the first attempt to system- atically investigate matrix forms of the Pascal triangle as mathematical objects in their own right.
Section 1.2 presents the four circulant operators and the way they intervene in generating the set of the twelve Pascal matrices, starting with one of them.
Section 3 generalize to the case of any square matrix.
1.2 The circulant operators
In this section, the circulant operators are presented as transformations of the generic matrix subscript vector (i, j) (0 ≤ i, j ≤ n).
Definition 6 - Circulant operator
The α-circulant operator i , j α
−−−→ i , i + j
mod n + 1 The β-circulant operator
i , j β
−−−→ −1 + i − j , j
mod n + 1 The γ-circulant operator
i , j γ
−−−→ i − j , j
mod n + 1 The δ-circulant operator
i , j δ
−−−→ i , 1 + i + j
mod n + 1
Figure 1 illustrates how α transforms a square matrix.
α
Figure 1: The α-circulant operator
Circulant transformations carry circulant matrices to associated row- or column-constant matrices. To see this, consider the circulant matrix C given be- low and its associated row-constant matrix denoted C: ˜
C =
c 1 c 2 c 3 · · · c n c n c 1 c 2 · · · c n − 1
c n−1 c n c 1 · · · c n−2 .. . .. . .. . . .. .. . c 2 c 3 c 4 · · · c 1
,
C ˜ =
c n c n c n · · · c n c n−1 c n−1 c n−1 · · · c n−1 c n − 2 c n − 2 c n − 2 · · · c n − 2
.. . .. . .. . .. .
c 1 c 1 c 1 · · · c 1
One readily sees for example that: C ˜ = βC.
1.3 Circulant Operators action on G n Theorem 7 Action of circulant operators on G n
If i ∈ {1, 5, 9} then βG i,n = G i+1 [12],n If i ∈ {2, 6, 10} then δG i,n = G i+1 [12],n If i ∈ {3, 7, 11} then γG i,n = G i+1 [12],n If i ∈ {4, 8, 12} then αG i,n = G i+1 [12],n where i + 1 [12] ≡ i + 1 mod 12.
From this stage one can now derived all the twelve G- matrices starting with any particular one. Thus start- ing with G 1,n , the twelve G-matrices can be derived in their natural order by the following sequence:
G 1,n −−−→ β G 2,n −−−→ δ G 3,n −−−→ γ G 4,n −−−→ α G 5,n
G 5,n −−−→ β G 6,n −−−→ δ G 7,n −−−→ γ G 8,n −−−→ α G 9,n
G 9,n −−−→ β G 10,n −−−→ δ G 11,n −−−→ γ G 12,n −−−→ α G 1,n This is a cycle since (αγδβ) 3 G 1,n = G 1,n . Moreover, it is readily verified that {G 1,n , G 5,n , G 9,n } is glob- ally (αγδβ)-invariant.
Similar inspections show that: (γδβα) 3 G 4,n = G 4,n , (δβαγ) 3 G 3,n = G 3,n , (βαγδ) 3 G 2,n = G 2,n , The circulant transformations provide a useful frame- work for investigating the G-matrices. Section 2 gives some basic properties of these operators which arise in the p-factor compositions. In section 3 we will extend their application to square matrices to provide new insights into the structure of the space of square matrices.
The set of circulant operators will be denoted by C n in dimension n + 1.
2 Main algebraic properties of the circulant operators
In this section the set C n is consider as an alphabet.
The aim here is to give a complete list of identities
which arise in the p-factor products (composition) of circulant operators (p = 1, 2, 3).
2.1 Notations and Definitions Let’s put forward these notations first:
< α >=
ǫ, α, α 2 , α 3 , . . . , α n
< β >=
ǫ, β, β 2 , β 3 , . . . , β n
< δ >=
ǫ, δ, δ 2 , δ 3 , . . . , δ n
< γ >=
ǫ, γ, γ 2 , γ 3 , . . . , γ n
< ξ > ⋄ =
ξ, ξ 2 , ξ 3 , . . . , ξ n ∀ξ ∈ C n
with ǫ = α n+1 = β n+1 = δ n+1 = γ n+1 and ǫM = M for all (n + 1) × (n + 1) matrix.
Definition 8 - Empty string
ε is called empty string and ∀ξ ∈ C n , ξ 0 ≡ ε Definition 9 - k-circulant operator
An operator ξ is k-circulant if:
∃ ω ∈ C n , such that ξ = ω k Theorem 10 For all ξ, ψ ∈ C n ,
< ξ > ⋄ ∩ < ψ > ⋄ = ∅ (15) 2.2 Identities without k -circulants
This section present all identities of exactly p-factors strings for p = 2, 3, 4 without k-circulant as sub- string.
2.3 Two-factors identities Theorem 11
αδ = δα βγ = γβ (16)
Proof:
It is easy to see that:
αδ(i, j) = α[δ(i, j)]
= α[(i, 1 + i + j) mod (n + 1)]
= α[(i, 1 + i + j)] mod (n + 1)
= (i, 1 + 2i + j) mod (n + 1)
and
δα(i, j) = δ[α(i, j)]
= δ[(i, i + j) mod (n + 1)]
= δ[(i, i + j)] mod (n + 1)
= (i, 1 + 2i + j) mod (n + 1) This leads to αδ = δα.
The second identity is also established in the same way.
2.4 Three-factors identities
We give above a collection of twelve identities.
αβα = βαβ αγα = γαγ (17) αδβ = δαβ αδγ = δαγ (18) αγδ = βαγ αγδ = δβα (19) αγδ = γδβ βαγ = δβα (20) βαγ = γδβ βδβ = δβδ (21) δβα = γδβ δγδ = γδγ (22) One of the main results deriving from these identities is:
Theorem 12 -
r ow- c olumn- r ow= c olumn- r ow- c olumn For all r ∈ {α, γ} and all c ∈ {β, δ},
rcr = crc
Proof:
Using the same technic as for equations 16, it is easy to verify that:
αβα = βαβ and αγα = γαγ on the one hand βδβ = δβδ and δγδ = γδγ on the other hand.
Another result is provided by:
Theorem 13
αγ δ = βαγ = δβα = γδβ
Proof:
It directly follows using the definitions.
These identities give the base on which all other p- factor identities (p > 3) will be derived. From this stage we now give some relevant identities with k- circulant operators for k = 1, 2, 3.
2.5 k -circulants identities 2.5.1 Two-factors for k = 2
α 2 δ 2 = δ 2 α 2 β 2 γ 2 = γ 2 β 2 (23) 2.5.2 Three-factors for k = 2
α 2 δ 2 β 2 = δ 2 α 2 β 2 α 2 δ 2 γ 2 = δ 2 α 2 γ 2 (24) β 2 γ 2 α 2 = γ 2 β 2 α 2 β 2 γ 2 δ 2 = γ 2 β 2 δ 2 (25) 2.5.3 Two-factors for k = 3
α 3 δ 3 = δ 3 α 3 β 3 γ 3 = γ 3 β 3 (26) 2.5.4 Three-factors for k = 3
α 3 δ 3 β 3 = δ 3 α 3 β 3 α 3 δ 3 γ 3 = δ 3 α 3 γ 3 (27) β 3 γ 3 α 3 = γ 3 β 3 α 3 β 3 γ 3 δ 3 = γ 3 β 3 δ 3 (28) 2.6 Identities involving k -circulant and cir-
culant operators ( k ≥ 2 )
We give here a theorem which can be considered as a basic tool to derive identities involving both k- circulant and circulant operators.
Theorem 14
α k δ k = δ k α k β k γ k = γ k β k (29) αδ k = δ k α βγ k = γ k β (30) α k δ = δα k β k γ = γβ k (31) Proof:
One can prove the first one (for example) by recur- rence.
Assuming that α k δ k = δ k α k for a k ∈ IN, α n+1 δ k+1 = α k αδδ k
= α k δαδδ k − 1
= α k δ 2 αδ k − 1
= . . .
= α k δ p αδ k − p+1
= . . .
= α k δ k αδ
= δ k α k δα
= δ k α k − 1 αδα
= δ k α k−1 δα 2
= . . .
= δ k α k − p+1 δα p
= . . .
= δ k αδα k
= δ k+1 α k+1
or:
α k+1 δ k+1 = αα k δ k δ
= αδ k α k δ
= αδδ k−1 α k−1 αδ
= δαδ k−1 α k−1 δα
= . . .
= δ p αδ k−p α k−p δα p
= . . .
= δ k αδα k
= δ k δαα k
= δ k+1 α k+1 On the other side:
αδ k = (αδ)δ k−1
= δαδ k−1
= δ(αδ)δ k − 2
= δ 2 αδ k−2 ...
= δ p αδ k − p ...
= δ k α This ends the proof.
3 Applications and generalizations
3.1 Triangular bipartition of a square matrix Now, the four triangular bipartitions of any square matrix as illustrated in figure 2 are presented. The triangular sub-block in full line indicate which block includes the main or anti diagonal.
SE/nw SW/ne
N W/se N E/sw
Figure 2: Four bipartitions of a square matrix
Definition 15 - Triangular bipartitions
Given A, a (n + 1) × (n + 1) matrix with subscripts i and j ranging from 0 to n.
(i) The North-East/south-west bipartition The matrices A N E and A sw , such that:
[A N E ] ij =
[A] ij if i ≤ j 0 if i > j and
[A sw ] ij =
[A] ij if i > j 0 if i ≤ j
define the North-East/south-west bipartition of A and A = A N E + A sw ≡ A N E/sw
(ii) The South-West/north-east bipartition The matrices A SW and A ne , such that:
[A SW ] ij =
[A] ij if i ≥ j 0 if i < j and
[A ne ] ij =
[A] ij if i < j 0 if i ≥ j
define the South-West/north-east bipartition of A and A = A SW + A ne ≡ A SW/ne
(iii) The North-West/south-east bipartition The matrices A N W and A se , such that:
[A N W ] ij =
[A] ij if i + j ≤ n 0 if i + j > n and
[A se ] ij =
[A] ij if i + j > n 0 if i + j ≤ n
define the North-West/south-east bipartition of A and A = A N W + A se ≡ A N W/se
(iv) The South-East/north-west bipartition The matrices A SE and A nw , such that:
[A SE ] ij =
[A] ij if i + j ≥ n 0 if i + j < n and
[A nw ] ij =
[A] ij if i + j < n 0 if i + j ≥ n
define the South-East/north-west bipartition of A and A = A SE + A nw ≡ A SE/nw
Theorem 16 α A N W/se
= α (A N W ) + α (A se ) (32) β A N E/sw
= β (A N E ) + β (A sw ) (33) δ A SE/nw
= δ (A SE ) + δ (A nw ) (34) γ A SW/ne
= γ (A SW ) + γ (A ne ) (35)
Subsets M atrices G n N E/sw G 1,n G 5,n G 9,n G n SE/nw G 2,n G 6,n G 10,n G n SW/ne G 3,n G 7,n G 11,n G n N W/se G 4,n G 8,n G 12,n
Table 1: Partition of the set G n
Proof:
It is derived from the fact that:
− A B and A b are triangular matrices (juste as G-matrices),
− circulant operators, if judiciously applied, doesn’t mixe two triangular blocks in a square matrix as we have seen it in the case of G-matrices.
3.2 Partition of the set G n with the circulant operators and generalization
The four bipartitions of the square matrix leads to a partition of G n into four subsets as it is described in table 1. One can observe that the subsets in the first column of this table (up to down) are globally invari- ant by the action of αγδβ, βαγδ, δβαγ and γδβα respectively.
Globally , it follows that:
G n N E/sw −−−→ G β n SE/nw −−−→ G δ n SW/ne −−−→ γ G n N W/se α
−−−→ G n N E/sw
β γ
δ α
Figure 3: Four bipartitions linked by circulant opera- tors
where −−−−→ O denotes the transformation by the operator O as shown in figure 3 which summarizes the generalization to the case of any square matrix A shown in table 2, where OA i,n = A i+1 [12],n and A 1,n = A, with:
i + 1 [12] = i + 1 modulus 12,
Subsets M atrices A N E/sw n A 1,n A 5,n A 9,n A SE/nw n A 2,n A 6,n A 10,n A SW/ne n A 3,n A 7,n A 11,n A N W/se n A 4,n A 8,n A 12,n Table 2: Partition of the set A n
and O =
β if i ∈ {1, 5, 9}
δ if i ∈ {2, 6, 10}
γ if i ∈ {3, 7, 11}
α if i ∈ {4, 8, 12}
Thus:
A 1,n −−−→ β A 2,n −−−→ δ A 3,n −−−→ γ A 4,n −−−→ α A 5,n
A 5,n −−−→ β A 6,n −−−→ δ A 7,n −−−→ γ A 8,n −−−→ α A 9,n
A 9,n −−−→ β A 10,n −−−→ δ A 11,n −−−→ γ A 12,n −−−→ α A 1,n This leads to the set A n of twelve new matrices from the single matrix A. Figure 4 shows a graph with A n as set of vertices, and the circulant operators as transitions labels.
A1,n A2,n A3,n A4,n
A5,n A6,n
A7,n
A8,n A12,n
A9,n A11,n
A10,n
Figure 4: Ring behavior of A n with alphabet in {α, β, γ, δ}
4 New structure of the space of square matrices
Let A = [A] i,j
0 ≤ i,j ≤ n be a square matrix:
- the set of the coefficients of A will be denoted by Coef (A),
- the permutation group on {(i, j), 0 ≤ i, j ≤ n} will be denoted by P erm(n),
- For σ ∈ P erm(n):
σ((i, j)) ≡ (i σ , j σ )
[A] i
σ,j
σ≡ [σA] i,j , 0 ≤ i, j ≤ n, σA n = {σA, A ∈ A n }
A k,n = { σA k,n , σ ∈ P erm (n)} where 1 ≤ k ≤ 12 Definition 17 - A permutation σ ∈ P erm(n) pre- serves a partition A B/b if their exists σ B and σ b such that
σ = σ B σ b where :
[A b ] i
σB
,j
σB= [A b ] i,j ∀ i, j [A B ] i
σb