Applied Mathematics
Vol.5 No.14(2014), Article
ID:48161,7
pages
DOI:10.4236/am.2014.514207
The Construction of Pairwise Additive Minimal BIB Designs with Asymptotic Results
Kazuki Matsubara1, Sanpei Kageyama2
1Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Japan
2Hiroshima Institute of Technology, Hiroshima, Japan
Email: d122307@hiroshima-u.ac.jp, s.kageyama.4b@it-hiroshima.ac.jp
Copyright © 2014 by authors and Scientific Research Publishing Inc.
This work is licensed under the Creative Commons Attribution International License (CC BY).
http://creativecommons.org/licenses/by/4.0/
Received 21 April 2014; revised 28 May 2014; accepted 12 June 2014
Abstract
An asymptotic existence of balanced incomplete block (BIB) designs and pairwise balanced designs (PBD) has been discussed in [1] -[3] . On the other hand, the existence of additive BIB designs and pairwise additive BIB designs with and
has been discussed with direct and recursive constructions in [4] -[8] . In this paper, an asymptotic existence of pairwise additive BIB designs is proved by use of Wilson’s theorem on PBD, and also for some
and
the exact existence of
pairwise additive BIB designs with block size
and
is discussed.
Keywords:Incidence Matrix, Pairwise Balanced Design (PBD), Balanced Incomplete Block Design (BIBD), Additive BIB Design, Pairwise Additive BIB Design, Wilson’s Theorem
1. Introduction
A pairwise balanced design (PBD) of order with block sizes in a set
is a system
, where
is a finite set (the point set) of cardinality
and
is a family of subsets (blocks) of
such that 1) if
, then
and 2) every pair of distinct elements of
occurs in
blocks of
[9] . This is denoted by
. When
, a
is especially called a balanced incomplete block (BIB) design, where
, each block contains
different points and each point appears in
different blocks [10] . This is denoted by
or
. It is well known that necessary conditions for the existence of a
are
. (1.1)
Let be the
incidence matrix of a BIB design, where
or 0 for all
and
, according as the i-th point occurs in the j-th block or otherwise. Hence the incidence matrix
satisfies the conditions: 1)
for all
, 2)
for all
, 3)
for all
.
Let, where
need not be an integer unlike other parameters. Further let
. A set of
is called
pairwise additive BIB designs if
corresponding incidence matrices
of the BIB design satisfy that
is the incidence matrix of a BIBD(
) for any distinct
. When
, this is especially called additive BIB designs [6] [7] .
It is clear by the definition that the existence of pairwise additive
implies the existence of
pairwise additive
for any
. Hence, for given parameters
, the larger
is, the more difficult a construction problem of
pairwise additive BIB designs is.
In pairwise additive, since a sum of any two incidence matrices yields a BIB design, it is seen [7] that
(1.2)
It follows from (1.2) that the existence of pairwise additive
implies
Pairwise additive are said to be minimal if
or
according as
is odd or even.
Some classes of pairwise additive
are constructed in [4] -[8] . It is clear by the definition that
. The purpose of this paper is to show that, for a given odd prime power
and a given positive integer
, the necessary conditions (1.1) for the existence of
pairwise additive minimal
are asymptotically sufficient on
. In particular, for the existence of
pairwise additive minimal
, (1.1) is asymptotically sufficient, i.e., there are
pairwise additive minimal
for sufficiently larger
, even if
. Furthermore, as the exact existence, it is shown that there are 2 pairwise additive
for any positive integer
except possibly for 12 values.
2.
The existence of is reviewed along with necessary and asymptotically sufficient conditions.
Let be a set of positive integers and
Necessary conditions for the existence of a are known as follows.
Lemma 2.1 [2] Necessary conditions for the existence of a are
(2.1)
Wilson [3] proved the asymptotic existence as Theorem 2.2 below shows.
Theorem 2.2 The necessary conditions (2.1) for the existence of a are asymptotically sufficient.
For any set of positive integers and any positive integer
, let
denote the smallest integer such that there are
for every integer
satisfying (2.1). Then Theorem 2.2 states the existence of
. On the other hand, some explicit bound for
was provided as follows.
Lemma 2.3 [11] There are for all positive integers
.
Especially, for a set being a set of prime powers of form
,
is shown as follows.
Lemma 2.4 ([12] Theorem 19.69) Let be a set of prime powers of form
with a positive integer
. Then there are
for all positive integers
, except possibly for
.
3. Construction by
In this section, a method of constructing pairwise additive BIB designs through is provided.
The following simple method is useful to construct pairwise additive BIB designs.
Lemma 3.1 The existence of a and
pairwise additive
for any
implies the existence of
pairwise additive
.
Proof. Let be the
and
,
. On the set
, let a block set
with all block size
be formed by the
pairwise additive
for each
. Then it follows that the
is the required BIB design.
For example, Lemma 3.1 yields the following.
Theorem 3.2 There are 4 pairwise additive for any integer
.
Proof. It follows from the fact ([4] [6] ) that there are additive, 4 pairwise additive
and additive
. Hence Lemmas 2.3 and 3.1 can yield the required designs.
As the next case of block sizes, is considered. A concept of
pairwise additive
has been discussed as a compatibly nested minimal partition in [12] , which shows the existence of pairwise additive
as follows.
Lemma 3.3 ([12] ; Theorem 22.12) Let be an odd prime power for a positive integer
. Then there are
pairwise additive
.
Lemmas 2.4, 3.1 and 3.3 can produce the following.
Theorem 3.4 ([12] ; Theorem 22.13) There are 2 pairwise additive for all positive integers
, except possibly for
.
Theorem 3.4 will be improved as in Theorem 6.7.
4. Some Class of Pairwise Additive
In this section, a necessary condition for the existence of pairwise additive being minimal is provided and then some classes of
pairwise additive
and (
pairwise) additive
are constructed.
Now (1.1) implies that necessary conditions for the existence of pairwise additive are
Furthermore, the following is given.
Theorem 4.1 When is an odd prime power, necessary conditions for the existence of pairwise additive
are
(4.1)
Proof. Since and
, when
is an odd prime power, it is shown that either
or
. Hence
implies
.
When is an odd prime power, a class of pairwise additive
is obtained as follows. This observation shows a generalization of Lemma 3.3.
Theorem 4.2 Let both and
be odd prime powers for a positive integer
. Then there are
pairwise additive
.
Proof. It can be shown that a development of the following initial blocks on yields incidence matrices
of the required BIB design:
where is a primitive element of
and
.
Furthermore, the following is known to be provided by recursive constructions with affine resolvable BIB designs. This result will be used in the next section.
Theorem 4.3 [7] Let be an odd prime power. Then there are additive
.
Especially, when, the further result is known.
Theorem 4.4 [8] There are additive B for any positive integer
.
5. Asymptotic Existence of Pairwise Additive Minimal
In this section, when is an odd prime power, an asymptotic existence of pairwise additive
is discussed, and it is shown that the necessary conditions (4.1) for the existence of
pairwise additive
are asymptotically sufficient for a given positive integer
.
Dirichlet’s theorem on primes is useful for the present discussion.
Theorem 5.1 (Dirichlet) If, then a set of integers of the following form
contains infinitely many primes.
Now Theorem 5.1 yields the following.
Lemma 5.2 [13] For any positive even integer, there are primes
and
for which
and
.
In the proof of Lemma 5.2 (i.e., Lemma 3.4 in [13] ), primes and
are obtained by using Theorem 5.1. Thus Lemma 5.2 implies the existence of sufficiently large primes
and
as follows.
Lemma 5.3 For a given odd prime power, there are primes
and
such that (a)
, (b)
, (c)
and (d)
for
.
Proof. Let be an odd prime power. Then, for an even integer
, Lemma 5.2 provides primes
and
such that (a)
, (b)
and
. Hence it is seen that
,
and
.
Now let. Then
which imply (c) and (d).
Thus one of the main results of this paper is now obtained through conditions (a), (b), (c) and (d) given in Lemma 5.3.
Theorem 5.4 For a given odd prime power, (4.1) is a necessary and asymptotically sufficient condition for the existence of
pairwise additive B
.
Proof (sufficiency). Let and
be primes as in Lemma 5.3 with
. Then conditions (c) and (d) show that there are
for sufficiently large
satisfying (4.1), on account of Theorem 2.2. Conditions (a) and (b) show that there are
pairwise additive
,
pairwise additive
and additive
, on account of Theorems 4.2 and 4.3. Hence the required designs can be obtained on account of Lemma 3.1.
Unfortunately, by use of Theorem 5.4 we cannot show the existence of pairwise additive
for
, since an additive
means
pairwise additive
.
Next, for a given odd prime power and a given positive integer
, even if
, the existence of
pairwise additive is discussed for sufficiently large
.
Lemma 5.5 For a given odd prime power and a given positive integer
, there are primes
and
such that (a)
, (b)
and (c)
.
Proof. Let be an odd prime power and
be a positive integer. Then, for a positive integer
, Lemma 5.2 provides primes p and q such that (a) p > q >
, (b)
and
. Hence it is seen that
and (c) holds.
Thus the following result is obtained through conditions (a), (b) and (c) as in Lemma 5.5.
Theorem 5.6 For a given odd prime power and a given positive integer
, there are
pairwise additive
for sufficiently large
.
Proof. Let and
be primes as in Lemma 5.5. Then it follows from (c) that there are
for sufficiently large, on account of Theorem 2.2. Also Theorem 4.2 along with conditions (a) and (b) shows that there are
pairwise additive
and
pairwise additive
. Thus the required designs are obtained on account of Lemma 3.1.
6. Pairwise Additive
In this section, the existence of pairwise additive is discussed. At first it is shown that there are
pairwise additive
for sufficiently large
, even if
. Furthermore, the exact existence of 2 pairwise additive
with
is discussed by providing direct and recursive constructions of pairwise additive
. Finally, it is shown that there are 2 pairwise additive
for any
except possibly for 12 values.
Three classes of pairwise additive are given as in Lemma 3.3 and Theorems 3.4 and 4.4. For
, 15 is the smallest value of
for which the existence of 2 pairwise additive
is unknown in literature. Hence at first this case is individually considered here.
Lemma 6.1 There are 2 pairwise additive.
Proof. It can be shown that a development of the following initial blocks on with the index being fixed yields incidence matrices
of the required BIB design:
with 15 elements. Here, in general
.
Now, pairwise additive
with sufficiently large
are obtained as follows. This shows an extension of Theorem 5.4 with
.
Theorem 6.2 For a given positive integer, even if
, there are
pairwise additive
with sufficiently large
.
Proof. Let be a positive integer satisfying
. Then (
pairwise) additive
are constructed by Theorem 4.4, and there are primes
and
such that
,
and
, on account of Lemma 5.2. Furthermore, since
and
with
, there are
for sufficiently large
, on account of Theorem 2.2. Hence
pairwise additive
for sufficiently large
can be constructed by Lemma 3.1 with
pairwise additive
,
pairwise additive
and additive.
Next, some recursive constructions of pairwise additive are provided. A combinatorial structure is here introduced. A transversal design, denoted by TD
, is a triple
such that 1)
is a set of
elements, 2)
is a partition of
into
classes (groups), each of size
, 3)
is a family of k-subsets (blocks) of
, 4) every unordered pair of elements from the same group is not contained in any block, and 5) every unordered pair of elements from other groups is contained in exactly
blocks. When
, we simply write
, where
[14] .
Since it is known [14] that the existence of a is equivalent to the existence of
mutually orthogonal latin squares of order
, the following result can be obtained, when
.
Lemma 6.3 [14] There exists a for all
except for
and possibly for
.
A method of construction is presented, similarly to a recursive construction given in [4] , by use of.
Theorem 6.4 The existence of pairwise additive
,
pairwise additive
and a
implies the existence of
pairwise additive
.
Proof. Let,
, be block sets of
pairwise additive
and
pairwise additive
respectively as
and let,
and
, denote an element which occurs in both the m-th block of a
and the n-th group. Then it can be shown that the following
incidence matrices yield the required
pairwise additive BIB designs with
elements denoted by
for
and
:
where and
.
Another recursive method is presented.
Theorem 6.5 The existence of pairwise additive
,
pairwise additive
and a
implies the existence of
pairwise additive
.
Proof. Let,
, be a block set similarly to the proof of Theorem 6.4 and let
,
, be a block set of
pairwise additive
, where
with elements
and
. Also let
,
and
, denote an element which occurs in both the m-th block of a
and the n-th group. Then the following
incidence matrices can yield the required
pairwise additive BIB designs with
elements denoted by
for
and
, and
:
where and
.
Now 2 pairwise additive are more obtained.
Lemma 6.6 There are 2 pairwise additive for
.
Proof. For, Theorem 6.5 with
provides the required BIB designs respectively, because 2 pairwise additive and 2 pairwise additive
are obtained by use of Theorems 3.4 and 4.4 and Lemma 6.1, and a
is also obtained by Lemma 6.3.
Hence on account of Lemma 6.6, the following result can be obtained. This improves Theorem 3.4.
Theorem 6.7 There are 2 pairwise additive for any positive integer
, except possibly for
.
Unfortunately, we cannot clear such 12 values displayed in Theorem 6.7. Furthermore, the existence of 2 pairwise additive has not been known except for
being
and 15 in Theorem 4.4 and Lemma 6.1.
Remark. Since Theorem 4.2 can be valid for a given odd integer, Theorem 5.6 is extended for a given odd integer
. On the other hand, when
is an even prime power, an asymptotic existence of pairwise additive minimal
is proved by some methods similar to Theorems 4.2, 4.3, 5.4 and 5.6. In particular, for
, the complete existence of
pairwise additive
has been shown in [4] [5] . However, in general, the exact existence of
pairwise additive minimal
with (1.1) could not be shown in this paper.
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