The combination of Ribet's theorem and the Taniyama-Shimura conjecture allowed the discovery of the proof of his most famous theorem by Andrew Wiles.
This thinker's Stationary Points Theorem states that if a function has a local extremum at some point and is differentiable there, then the function's derivative at that point must be zero. This man proved that an integer raised to the power of a prime is congruent to itself, modulo the prime, in his(*) Little Theorem. The combination of Ribet's theorem and the Taniyama-Shimura conjecture allowed the discovery of the proof of his most famous theorem by Andrew Wiles. For 10 points, name this 17th Century French mathematician who claimed that a proof of his Last Theorem was too large to fit in the margin of his book.
This thinker's Stationary Points Theorem states that if a function has a local extremum at some point and is differentiable there, then the function's derivative at that point must be zero.
Pierre de Fermat