30 2. Basic notions of representation theory spread of his mental illness, possibly fueled by his opponents, who tried to invalidate his accusations. In the meantime, trying to assert its cultural (and eventually political) independence from Sweden, Norway took steps to bring back its leading intellectuals. The Norwegian National Assembly voted to establish a personal chair in transformation group theory for Lie, matching his high Leipzig salary. Lie was anxious to return to his homeland, but his wife and three children did not share his nostalgia. He eventually returned to Norway in 1898 with only a few months to live. Lie “thought and wrote in grandiose terms, in a style that has now gone out of fashion, and that would be censored by our scientific journals”, wrote one commentator [26, p. iii]. Lie was always more concerned with originality than with rigor. “Let us reason with con- cepts!” he often exclaimed during his lectures and drew geometrical pictures instead of providing analytical proofs [22, p. 244]. “With- out Phantasy one would never become a Mathematician”, he wrote. “[W]hat gave me a Place among the Mathematicians of our Day, despite my Lack of Knowledge and Form, was the Audacity of my Thinking” [56, p. 409]. Hardly lacking relevant knowledge, Lie indeed had trouble putting his ideas into publishable form. Due to Engel’s diligence, Lie’s research on transformation groups was summed up in three grand volumes, but Lie never liked this ghost-written work and preferred citing his own earlier papers [47, p. 310]. He had even less luck with the choice of assistant to write up results on contact transformations and partial differential equations. Felix Hausdorff’s interests led him elsewhere, and Lie’s thoughts on these subjects were never completely spelled out [16, p. 324]. Thus we may never discover the “true Lie”. 2.11. Tensor products In this subsection we recall the notion of tensor product of vector spaces, which will be extensively used below. Definition 2.11.1. The tensor product V ⊗ W of vector spaces V and W over a field k is the quotient of the space V ∗ W whose basis

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