For example, the definition given by Felix Hausdorff in 1914 is equivalent to the modern definition plus the Hausdorff separation axiom.
Felix Mendelssohn | Felix Frankfurter | Felix the Cat | Félix Guattari | María Félix | Félix Trinidad | Felix Klein | Julie Felix | Felix Rodriguez | Félix Houphouët-Boigny | Felix Da Housecat | José Félix Ribas Municipality, Aragua | Felix von Luckner | Felix Unger | Félix Rodríguez (Central Intelligence Agency) | Felix Magath | Félix Fénéon | Allyson Felix | Felix Werder | Felix the Cat: The Movie | Félix Resurrección Hidalgo | Felix Dzerzhinsky | Felix da Housecat | Felix Cavaliere | Rachel Félix | Felix Silla | Félix Savón | Felix Nussbaum | Felix Mottl | Felix Gonzalez-Torres |
For the study of astronomical refraction he developed an unusual method of calculating the vertical gradient of air temperature together with his Assistent Felix Hausdorff.