Determining the "toughest" theorem in mathematics can be quite subjective, as the difficulty of a theorem can depend on various factors, including the background knowledge of the individual mathematician and the context in which the theorem is being studied. However, several theorems are widely recognized for their complexity and the profound challenges they present. Here are a few that are often cited:
### 1. **Fermat's Last Theorem**
- **Statement:** For any integer \( n > 2 \), there are no three positive integers \( a \), \( b \), and \( c \) that satisfy the equation \( a^n + b^n = c^n \).
- **Difficulty:** Proposed by Pierre de Fermat in 1637, this conjecture was famously noted in the margin of his copy of an ancient Greek text, with Fermat claiming he had a "truly marvelous proof" that the margin was too narrow to contain. It wasn't proven until 1994 by Andrew Wiles, with the help of Richard Taylor. The proof involves advanced concepts from algebraic geometry and number theory, specifically modular forms and elliptic curves. The difficulty lay not just in proving it, but in developing the mathematical tools necessary to do so.
### 2. **The Riemann Hypothesis**
- **Statement:** All non-trivial zeros of the Riemann zeta function, a complex function defined for complex numbers, lie on the critical line where the real part is \( \frac{1}{2} \).
- **Difficulty:** Formulated by Bernhard Riemann in 1859, this hypothesis remains unproven. It has deep implications in number theory, particularly in the distribution of prime numbers. The difficulty of the Riemann Hypothesis lies in its complexity and the subtlety of the zeta function’s behavior. It’s central to many areas in mathematics, and proving or disproving it remains one of the most significant open problems in mathematics.
### 3. **The Hodge Conjecture**
- **Statement:** For a non-singular projective algebraic variety, certain classes in the cohomology group can be represented by algebraic cycles.
- **Difficulty:** Proposed by Sir Michael Atiyah and is still one of the major unsolved problems in algebraic geometry. The conjecture relates to the structure of complex algebraic varieties and has connections to many areas in mathematics. Proving it involves intricate knowledge of algebraic geometry, topology, and differential geometry.
### 4. **The P vs NP Problem**
- **Statement:** The question of whether every problem for which a proposed solution can be verified quickly (in polynomial time) can also be solved quickly (in polynomial time).
- **Difficulty:** This is one of the seven "Millennium Prize Problems" for which the Clay Mathematics Institute has offered a $1 million prize for a correct solution. The difficulty lies in understanding the relationship between the classes of problems that can be verified versus those that can be solved efficiently, a topic central to theoretical computer science and complexity theory.
### 5. **The Birch and Swinnerton-Dyer Conjecture**
- **Statement:** The conjecture provides a deep relationship between the number of rational points on an elliptic curve and the behavior of its L-function.
- **Difficulty:** Also one of the Millennium Prize Problems, this conjecture is central to the study of elliptic curves and arithmetic geometry. Its resolution requires a thorough understanding of various areas including number theory, algebraic geometry, and L-functions.
### Summary
These theorems and conjectures represent some of the most profound and challenging problems in mathematics. Each requires an intricate and deep understanding of different mathematical fields, and the difficulty of solving them often leads to new developments and insights within those fields.