Hello, Please excuse me about my ignorance. I would like to know how SO(2,1) Lie algebra, is derived from operators and commutators. I have some notes, that the Lie algebra of SO(2,1) is derived from: [D,H]=-iH [K,D]=-iK [H,K]=2iD where D, H, and K are the "generators". I have no clue what does the word "generators" mean, nor how commutators derive an algebra. Please let me know. Thank you, Askalot.