Karl Frithiof Sundman (28 October 1873, Kaskinen – 28 September 1949, Helsinki) was a Finnish mathematician who used analytic methods to prove the existence of a convergent infinite series solution to the three-body problem in 1906 and 1909.
He was awarded the Pontécoulant prize by the French Academy of Science in 1913 for this work. He also published a paper on regularization methods in mechanics in 1912.
In 1947, Sundman was elected a foreign member of the Royal Swedish Academy of Sciences.
The crater Sundman on the Moon is named after him, as is the asteroid 1424 Sundmania.