Checking $sum_beta in Phi^+ langle beta, alpha^vee rangle = 2$ for a simple root $alpha$
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Clash Royale CLAN TAG #URR8PPP up vote 0 down vote favorite Let $mathfrak g$ be a semisimple complex Lie algebra and $Phi$ a set of simple roots. Let $alpha in Phi$, I want to know why $$ sum_beta in Phi^+ langle beta, alpha^vee rangle = 2$$ I checked it by hands for $mathfrak sl_n$ for $n=2,3,4$. I don't know how to generalize and this is stated without proof in several places for arbitrary semisimple Lie algebra so I'm sure I miss something. Any hints is appreciated. lie-algebras root-systems share | cite | improve this question asked Jul 26 at 11:29 student 71 8 add a comment  | up vote 0 down vote favorite Let $mathfrak g$ be a semisimple complex Lie algebra and $Phi$ a set of simple roots. Let $alpha in Phi$, I want to know why $$ sum_beta in Phi^+ langle beta, alpha^vee rangle = 2$$ I checked it by hands for $mathfrak sl_n$ for $n=2,3,4$. I don't know how to generalize and this is stated without proof in s