Constantine Karatheodori was a Greek mathematician born in 1873 who made significant contributions to real analysis and calculus of variations. He created his own important theorem in the 20th century and worked with other great mathematicians of his time. Karatheodori excelled at languages, being fluent in Greek, French, German, English, Italian, Turkish, and ancient languages. His doctoral thesis on discontinuous solutions in calculus of variations was a pioneering work that developed the theory.