Which ancient Greek mathematician is known for a theorem regarding right triangles?

The ancient Greek mathematician known for a theorem regarding right triangles is Pythagoras. He is iconic for formulating what is now known as the Pythagorean Theorem, which states that in any right triangle, the square of the length of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the lengths of the other two sides. This theorem is fundamental in mathematics and geometry, illustrating a key relationship between the sides of a right triangle.

Pythagoras' contributions extend beyond this theorem; he also founded a school that studied mathematics, philosophy, and the natural world, underscoring the significance of his work within the larger context of ancient Greek thought. While other mathematicians mentioned, such as Euclid and Archimedes, made substantial contributions to mathematics, they are not specifically tied to this theorem regarding right triangles. Euclid is known for his work in geometry and laying down the axioms and postulates that form the basis of plane geometry, whereas Archimedes is noted for his contributions to calculus and the principles of mechanics.

Therefore, Pythagoras stands out as the individual directly associated with the specific theorem dealing with right triangles.