Lars Valter Hörmander (24 January 1931 – 25 November 2012) was a Swedish mathematician who has been called "the foremost contributor to the modern theory of linear partial differential equations".

In 2006 he was awarded the Steele Prize for Mathematical Exposition for his four-volume textbook Analysis of Linear Partial Differential Operators, which is considered a foundational work on the subject.

At the time when he entered the gymnasium, the principal had instituted an experiment of reducing the period of the education from three to two years, and the daily activities to three hours.

He turned to partial differential equations when Riesz retired and Lars Gårding who worked actively in that area was appointed professor.

These locations offered "much to learn" in partial differential equations, with the exception of Chicago of which he however notes the Antoni Zygmund seminar held by Elias Stein and Guido Weiss to have strengthened his familiarity with harmonic analysis.

Hörmander was briefly director of the Mittag-Leffler Institute in Stockholm between 1984 and 1986 but only accepted a two-year appointment as he "suspected that the administrative duties would not agree well" with him, and found that "the hunch was right".

A follow-up of his Linear Partial Differential Operators, "illustrate[d] the vast expansion of the subject"[4] over the past 20 years, and is considered the "standard of the field".

[5] In addition to these works, he has written a recognised introduction to several complex variables based on his 1964 Stanford lectures, and wrote the entries on differential equations in Nationalencyklopedin.