Most linear systems of ordinary differential equations can be solved only by techniques of approximation. The author discusses a method of geometric integration using rooted trees and graded Lie algebras. (pp. 430)
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Louis Nirenberg is one of the great analysts of the twentieth century. In this interview, he reminisces about his long career at the Courant Institute of Mathematical Sciences, some highlights of his research, and the joys of mathematical collaboration. (pp. 441)
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