proof explanation - Knapp, Lie Groups: beyond an Introduction, Theorem II.2.15 - Mathematics Stack Exchange,An Introduction to Lie Groups and Lie Algebras,Structure and Geometry of Lie Groups | SpringerLink,Orient BlackSwan,An Introduction to Lie Groups and Lie Algebras (Cambridge Studies in Advanced Mathematics, Series Number 113): Kirillov Jr Jr, Alexander: 9780521889698: Amazon.com: Books