CMSC203 Discrete Structures, Sections 0201, Spring 2008

⇐ Proofs by Contradiction Proof by Cases ⇒

Existence Proofs

An existence proof establishes the existence of a mathematical object with some properties we desire. Existence proofs may be constructive or non-constructive. A constructive proof explicitly specifies the object. Here, the specification can be mathematical --- infinite objects do not have to be written down. Non-constructive existence proofs only show that the desired object exists. Some examples will clarify this concept.

Example 1: In the section on proof by contradiction, we used a graph that was not 3-colorable. This graph has triangles --- 3 vertices that are connected to each other. For example, vertices a, e and c form a triangle:
It was very useful to have triangles in our argument that the graph is not 3-colorable, because the 3 vertices in a triangle have to be colored with 3 different colors. Are triangles necessary to prove that a graph is not 3-colorable? No.

Claim: There exists a triangle-free graph that is 4-colorable, but not 3-colorable.

Proof: Consider the following graph and its 4-coloring:
By checking every triple of vertices, the reader can verify that the graph is triangle-free. We leave the proof that the graph is not 3-colorable as an exercise.

Example 2: The proof in the previous example is definitely constructive. The graph with the desired property is given explicitly. In this example we prove the existence of a number with certain properties without saying which number has the property.

Claim: There exist irrational numbers x and y such that xy is rational.

Proof: Let z = √2 2 . If z is rational then z is our desired number with x = √2 and y = √2.

Now, suppose that z is irrational. Then, let x = z and y = √2.

xy = (√2 2 ) 2 = √2 ( √2 ⋅ √2 ) = √2 2 = 2.
In this case, xy is again rational.

In either case, whether z is rational or irrational, we've shown the existence of irrational numbers x and y such that xy is rational.


The proof in Example 2 is definitely non-constructive. The proof establishes the claim but does not tell us if it is √2 2 or (√2 2 ) 2 that is rational. Generally speaking, non-constructive proofs are less satisfying than constructive proofs. In computer science, especially, constructive existence proofs are preferred because we often want to use the desired object in a program or algorithm. A non-constructive proofs would not help there.
⇐ Proofs by Contradiction Proof by Cases ⇒

Last Modified: 14 Feb 2008 23:32:24 EST by Richard Chang
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