How to Master the World of Conjectures and Counterexamples

In math, a conjecture is like a smart guess — something we think is true but haven't proven. If someone finds an example that shows the guess is wrong, that's a counterexample. It's a bit like playing a detective game in mathematics. In this guide, we'll look at these two ideas, breaking them down in easy-to-understand terms.

How to Master the World of Conjectures and Counterexamples

Step-by-step Guide: Conjectures and Counterexamples

Understanding Conjectures:
A conjecture is an unproven statement that is believed to be true based on observations. Conjectures arise from patterns noticed by mathematicians. While some conjectures have been proven, others remain unproven and open to exploration.

Recognizing Counterexamples:
A counterexample is a specific case or instance that disproves a conjecture or statement. If even one counterexample exists, it means the conjecture is not universally true.

Counterexamples are indispensable in mathematics for several reasons:

  • They refine and correct conjectures.
  • They prevent mathematicians from pursuing false statements.
  • They offer clarity on the limitations of a statement’s accuracy.


Example 1: Conjecture about Prime Numbers
Conjecture: “All numbers less than \(10\) are prime.”

Considering the numbers less than \(10\): \(2\), \(3\), \(4\), \(5\), \(6\), \(7\), \(8\), and \(9\), we can identify counterexamples. Numbers \(4\), \(6\), \(8\), and \(9\) are not prime. Hence, the conjecture is false.

Example 2: Conjecture about Even Numbers
Conjecture: “All even numbers greater than \(2\) are not prime.”

For this conjecture, the number \(2\) is a counterexample. \(2\) is even and is also prime. While the statement does mention numbers greater than \(2\), the inclusion of \(2\) as an even prime number highlights the need for precision in the formulation of conjectures.

Example 3: Fermat’s Last Theorem
Conjecture: There are no three integers \(a\), \(b\), and \(c\) that can satisfy the equation \(a^n + b^n = c^n\) for any integer value of \(n\) greater than \(2\).

This conjecture remained unproven for centuries. However, it was eventually proven true by Andrew Wiles in 1994, meaning there are no counterexamples.

Practice Questions:

  1. Conjecture: All positive integers are greater than \(0\). Is this true or false? If false, provide a counterexample.
  2. Conjecture: The square of any integer is positive. Is this true or false? If false, provide a counterexample.
  3. Conjecture: All birds can fly. Is this true or false? If false, provide a counterexample.


  1. True. By definition, positive integers are greater than \(0\).
  2. True. The square of any integer, whether positive or negative, is always positive.
  3. False. Counterexample: Ostriches, penguins, and kiwis are birds that cannot fly.

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