The most prolific mathematician who ever lived — a Swiss master who worked at the academies of St Petersburg and Berlin and produced so much that roughly half of it came after he went nearly blind, dictated from memory to his sons while he held elaborate calculations in his head. He standardized much of the notation students still use: f(x), e, i, the summation sign, and the modern symbol for pi. He solved the seven bridges of Königsberg and opened graph theory, ranged across analysis, number theory, mechanics, and astronomy, and remained so productive that publication of his collected works continued for decades after his death in 1783.
He kept working after he could no longer see. In 1771 Leonhard Euler lost what little sight remained in his good eye, and from then on he could no longer read his own equations and worked through memory and scribes. It barely slowed him. Dictating to his sons and to assistants, holding vast calculations in his head, he produced roughly half of his life's work while effectively blind, and he was already the most prolific mathematician who had ever lived. He gave us the symbols students still write every day, opened new lines of mathematics while solving concrete problems, and ranged across nearly every field he touched. The output simply did not stop.