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Mar 30, 2015 at 19:19 history edited Dave Witte Morris CC BY-SA 3.0
proof of the reducible case was much more complicated than necessary
Mar 30, 2015 at 18:10 comment added Dave Witte Morris The proof in the first paragraph shows that $n-1$ is not sharp unless $n$ is very small. This is because $\dim \mathfrak{g} = \dim (\mathfrak{b} \cap \mathfrak{b}^-) + 2 \dim (\mathfrak{g}/\mathfrak{b})$ is larger than $1 + 2 \dim (\mathfrak{g}/\mathfrak{b})$ unless $\mathfrak{g} = \mathfrak{sl}_2(\mathbb{C})$ (so $\dim \mathfrak{g} = 3$).
Mar 30, 2015 at 18:03 history edited Dave Witte Morris CC BY-SA 3.0
correction: need an opposite Borel subalgebra to apply to y
Mar 30, 2015 at 17:37 history edited Dave Witte Morris CC BY-SA 3.0
corrected a typo and removed an ambiguity
Mar 30, 2015 at 17:31 history answered Dave Witte Morris CC BY-SA 3.0