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1
CHAPTER 1
INTRODUCTION TO BRT
1.1. General Formulation.
1.2. Some BRT Settings.
1.3. Complementation Theorems.
1.4. Thin Set Theorems.
1.1. General Formulation.
Before presenting the precise formulation of Boolean
Relation Theory (BRT), we give two examples of assertions
in BRT that are of special importance for the theory.
DEFINITION 1.1.1. N is the set of all nonnegative integers.
A\B = {x: x
∈
A
∧
x
∉
B}.
THIN SET THEOREM. Let k
≥
1 and f:N
k
→
N. There exists an
infinite set A
⊆
N such that f[A
k
]
≠
N.
COMPLEMENTATION THEOREM. Let k
≥
1 and f:N
k
→
N. Suppose
that for all x
∈
N
k
, f(x) > max(x). There exists an infinite
set A
⊆
N such that f[A
k
] = N\A.
These two theorems are assertions in BRT. In fact, the
complementation theorem has the following sharper form.
COMPLEMENTATION THEOREM (with uniqueness). Let k
≥
1 and
f:N
k
→
N. Suppose that for all x
∈
N
k
, f(x) > max(x). There
exists a unique set A
⊆
N such that f[A
k
] = N\A.
Furthermore, A is infinite.
We will explore the Thin Set Theorem and the
Complementation Theorem in sections 1.3, 1.4. At this point
we analyze their logical structure.
DEFINITION 1.1.2. A multivariate function on N is a
function whose domain is some N
k
and whose range is a subset
of N. A strictly dominating function on N is a multivariate
function on N such that for all x
∈
N
k
, f(x) > max(x). We
define MF as the set of all multivariate functions on N, SD
as the set of all strictly dominating functions on N, and
INF as the set of all infinite subsets of N.
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DEFINITION 1.1.3. Let f
∈
MF, where dom(f) = N
k
. For A
⊆
N,
we define fA = f[A
k
].
The notation fA is very convenient. It avoids the
unnecessary use of explicit mention of arity or dimension.
It is used throughout this book.
Using this notation, we can restate our two theorems as
follows.
THIN SET THEOREM. For all f
∈
MF there exists A
∈
INF such
that fA
≠
N.
COMPLEMENTATION THEOREM. For all f
∈
SD there exists A
∈
INF such that fA = N\A.
Note that in the Thin Set Theorem, we use the family of
multivariate functions MF, and the family of sets INF. In
the Complementation Theorem, we use the family of
multivariate functions SD, and the family of sets INF.
In BRT terminology this will be expressed by saying that
the Thin Set Theorem is an instance of IBRT (inequational
BRT) on the BRT setting (MF,INF), and the Complementation
Theorem is an instance of EBRT (equational BRT) on the BRT
setting (SD,INF).
Note that we can regard the condition fA
≠
N as a Boolean
inequation in fA,N. We also regard the condition fA = N\A
as a Boolean equation in fA,N.
Here N plays the role of the universal set in Boolean
algebra. From this perspective, fA
≠
N is a Boolean
inequation in fA, and fA = N\A is a Boolean equation in
A,fA.
The fact that N should play the role of the universal set
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 Fall '08
 JOSHUA
 Math

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