Let g be a simple Lie algebra.
Let ~Lg be the universal central extension of Lg:=g[t,t−1]. Let Vλ be a positive energy representation of ~Lg of level k and highest weight λ.
Then the minimal energy hλ of Vλ is given by the well-known formula
hλ=∥λ+ρ∥2−∥ρ∥22(k+g∨)
where ρ is the half-sum of all positive roots, and g∨ is the dual Coxeter number.
I am looking for a citable reference for the above formula, i.e., one that includes a proof.
Now, for the benefit of the reader, I will define the terms "positive energy representation" and "minimal energy".
First of all, the affine Kac-Moody algebra
~Lge=~Lg⋊CL0 has underlying vector space
~Lg⊕CL0=g[t,t−1]⊕Cc⊕CL0
and Lie bracket given by the requirements that
c is central and that
[tmX+aL0,tnY+bL0]=tm+n[X,Y]+mδm+n,0⟨X,Y⟩c−natnY+mbtmX.
Note that
L0 acts like
−tddt.
A representation V of ~Lg is called positive energy if the action of ~Lg on V can be extended (such an extension is never unique!) to an action of ~Lge in such a way that L0 acts with positive spectrum and finite dimensional eigenspaces.
To see that the extension is never unique, note that one can add an arbitrary multiple of the identity operator to L0, without destroying the commutation relations.
To make the extension unique, one considers the Lie algebra ~Lg⋊sl(2) instead, where the copy of sl(2) is spanned by elements L−1, L0, L1.
The action of Ln∈sl(2) on ~Lg is by −tn+1ddt.
It turns out that, if the action of ~Lg on V extends to ~Lg⋊CL0, then it always also extends to ~Lg⋊sl(2). However, among all the possible ways of extending the action to ~Lg⋊CL0, only one of them has the property that it further extends to ~Lg⋊sl(2).
The moral of the story is that there is a preferred way of extending the action of ~Lg on V to an action of ~Lg⋊CL0.
The minimal energy of the positive energy representation V is the smallest eigenvalue of L0.
Finally, for completeness, the central charge is the scalar by which the central element c∈~Lg acts.
This post imported from StackExchange MathOverflow at 2014-09-10 17:15 (UCT), posted by SE-user André Henriques