Complex sentence: Professor Marsden, together with Vladimir Igorevich Arnol'd and Alan Weinstein is one of the world leading authorities in mathematical and theoretical classical mechanics, who has laid much of the foundation for the symplectic/metaplectic theory.
Split into simpler sentences: Professor Marsden, together with Vladimir Igorevich Arnol'd and Alan Weinstein is one of the world leading authorities in mathematical and theoretical classical mechanics. He has laid much of the foundation for the symplectic/metaplectic theory.