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Theory of Statistics - George Mason University

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668 0 Statistical Mathematics<br />

or a “corollary”. While these terms have meanings, the meanings are rather<br />

vague or subjective, and many authors’ usage <strong>of</strong> the different terms serves<br />

no purpose other than to annoy the reader. If a proposition has no known<br />

pro<strong>of</strong>, it is sometimes called a “conjecture”. We usually do not use the term<br />

“proposition” to refer to a conditional statement that has been disproved.<br />

A declarative statement has one <strong>of</strong> two mutually exclusive states: “true”<br />

or “false”. We denote the negation or falsification <strong>of</strong> the statement A by ¬A.<br />

With a basic proposition, such as<br />

A ⇒ B,<br />

there are associated four related propositions:<br />

contrapositive,<br />

¬B ⇒ ¬A,<br />

inverse,<br />

converse,<br />

and contradiction,<br />

¬A ⇒ ¬B,<br />

B ⇒ A,<br />

¬(A ⇒ B).<br />

If a proposition is true, then its contraposition is also true, but its contradiction<br />

is not true. The inverse is the contrapositive <strong>of</strong> the converse. The<br />

contradiction <strong>of</strong> the contradiction <strong>of</strong> a proposition is the proposition. Within<br />

any mathematical system there are propositions which are neither true nor<br />

false.<br />

There are various types <strong>of</strong> pro<strong>of</strong>s for propositions. Some are “better” than<br />

others. (See Aigner and Ziegler (2010) for discussions <strong>of</strong> different types <strong>of</strong><br />

pro<strong>of</strong>.) The “best” pro<strong>of</strong> <strong>of</strong> a proposition is a direct pro<strong>of</strong>, which is a sequence<br />

<strong>of</strong> statements “if A then A1, if A1 . . ., . . . then B”, where each statement in<br />

the sequence is an axiom or a previously proven proposition. A direct pro<strong>of</strong> is<br />

called deductive, because each <strong>of</strong> the steps after the first is deduced from the<br />

preceding step.<br />

Occasionally, the Axiom <strong>of</strong> Choice is used in a pro<strong>of</strong>. This axiom, which<br />

we encountered on page 611, is outside the usual axiomatic basis <strong>of</strong> much <strong>of</strong><br />

mathematics. The Axiom <strong>of</strong> Choice basically says that given any collection<br />

<strong>of</strong> sets, even an infinite collection, it is possible to form a set consisting <strong>of</strong><br />

exactly one element from each set in the collection. The Axiom <strong>of</strong> Choice is<br />

tautological for a finite collection.<br />

Whenever the Axiom <strong>of</strong> Choice is used in a pro<strong>of</strong>, that fact should be<br />

stated. Also, whenever an indirect method <strong>of</strong> pro<strong>of</strong> is used, the type <strong>of</strong> the<br />

pro<strong>of</strong> should be stated or described.<br />

<strong>Theory</strong> <strong>of</strong> <strong>Statistics</strong> c○2000–2013 James E. Gentle

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