Leonhard who helped develop calculus

EULER

This Swiss mathematician also lent his name to a famous constant (approximately 2.718) and made groundbreaking contributions to graph theory.

This clue points to Leonhard Euler, a famous Swiss mathematician who made major contributions to calculus alongside Newton and Leibniz. The name "Leonhard" is given directly, so you just need to recall his last name—no wordplay or tricks here. Euler is a well-known figure in math, often called the "Shakespeare of mathematics" for his vast influence.