Ernst Leonard Lindelöf (Swedish pronunciation: [ˈlɪ̂nːdɛˌløːv]; 7 March 1870 – 4 June 1946) was a Finnish mathematician, who made contributions in real analysis, complex analysis and topology. Lindelöf spaces are named after him. He was the son of mathematician Lorenz Leonard Lindelöf and brother of the philologist Uno Lorenz Lindelöf [fi].^[1]

He was secretary of the Finnish Society of Science and Letters (societas scientiarum Fennica) in its centenary year, 1938. ^[2]

Lindelöf studied at the University of Helsinki, where he completed his PhD in 1893, became a docent in 1895 and professor of Mathematics in 1903. He was a member of the Finnish Society of Sciences and Letters.

In addition to working in a number of different mathematical domains including complex analysis, conformal mappings, topology, ordinary differential equations and the gamma function, Lindelöf promoted the study of the history of Finnish mathematics. He is known for the Picard–Lindelöf theorem on differential equations and the Phragmén–Lindelöf principle, one of several refinements of the maximum modulus principle that he proved in complex function theory. He was the PhD supervisor for Lars Ahlfors at the University of Helsinki.

• Le calcul des résidus et ses applications à la théorie des fonctions (Paris, 1905)
• Mémoire sur la théorie des fonctions entières d'ordre fini ("Acta societatis scientiarum fennicae" 31, 1903)
• With Lars Edvard Phragmén: "Sur une extension d'un principe classique de l'analyse et sur quelques propriétés des fonctions monogènes dans le voisinage d'un point singulier", in: Acta Mathematica 31, 1908.