Petersen and his colleague Zeuthen, the two professors at the University of Copenhagen, established a mathematical tradition in Denmark almost out of nothing. Both are still remembered for their geometry, but Zeuthen also as a historian of mathematics and Petersen for the graph theory.

Petersen did pioneering work in several fields, including algebra, combinatorics, cryptography, economics, function theory, geometry, mathematical physics and the teaching of mathematics. But both his graph theory and some of the other brilliant pieces went unnoticed or met with outright rejection in his own time.This is not to imply that his life was one of disappointment - far from it. He was the embodiment of the best sense of humour and the most vigorous joy in life.

A biography of Petersen by Jesper Lutzen, Gert Sabidussi and Bjarne Toft has been published in Discrete Mathematics Volume 100. The talk is based on that paper and original material from the time. Links to present day graph theory will be made.

• It has an easy definition in terms of Riemann sums, not requiring any measure theory.
• Every derivative is Henstock integrable. This is not true of Riemann or Lebesgue integrals! Thus, we have the most complete version of the Fundamental Theorem of Calculus and Stokes's Theorem.
• The Henstock integral is nonabsolute. For a function to be integrable it is not necessary for its absolute value to be integrable.

Refreshments at 3:15 p.m. in Room 321 Altgeld Hall