indirect proof

Direct proof

In mathematics and logic, a direct proof is a way of showing the truth or falsehood of a given statement by a straightforward combination of established facts, usually existing lemmas and theorems, without making any further assumptions. In order to directly prove a conditional statement of the form "If p, then q", it is only necessary to consider situations where the statement p is true. Logical deduction is employed to reason from assumptions to conclusion. The type of logic employed is almost invariably first-order logic, employing the quantifiers for all and there exists. Common proof rules used are modus ponens and universal instantiation.

In contrast, an indirect proof may begin with certain hypothetical scenarios and then proceed to eliminate the uncertainties in each of these scenarios until an inescapable conclusion is forced. For example instead of showing directly p → q, one proves its contrapositive ~q → ~p (one assumes ~q and shows that it leads to ~p). Since p → q and ~q → ~p are equivalent by the principle of transposition, one has indirectly proved p → q. Proof methods that are not direct include Proof by contradiction, Proof by exhaustion, Proof by infinite descent and Proof by induction.

Example

What follows is a simple, direct proof that the sum of two even integers is itself an even number.

Consider two even integers x and y. Since they are even, they can be written as x=2a and y=2b respectively for integers a and b. Then the sum x+y = 2a + 2b = 2(a+b). From this it is clear x+y has 2 as a factor and therefore is even, so the sum of any two even integers is even.

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