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2.15: Appendix II- Legendre Transformations

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A convex function of a single variable f(x) is one for which f(x)>0 everywhere. The Legendre transform of a convex function f(x) is a function g(p) defined as follows. Let p be a real number, and consider the line y=px, as shown in Figure 2.15.1. We define the point x(p) as the value of x for which the difference F(x,p)=pxf(x) is greatest. Then define g(p)=F(x(p),p).26 The value x(p) is unique if f(x) is convex, since x(p) is determined by the equation

f(x(p))=p .

Note that from p=f(x(p)) we have, according to the chain rule,

ddpf(x(p))=f(x(p))x(p)x(p)=[f(x(p))]1 .

From this, we can prove that g(p) is itself convex:

g(p)=ddp[px(p)f(x(p))]=px(p)+x(p)f(x(p))x(p)=x(p) ,

hence

g(p)=x(p)=[f(x(p))]1>0 .

clipboard_e1024ab614a8717f55b8bbba63a60dfd5.png
Figure 2.15.1: Construction for the Legendre transformation of a function f(x).

In higher dimensions, the generalization of the definition f(x)>0 is that a function F(x1,,xn) is convex if the matrix of second derivatives, called the Hessian,

Hij(x)=2Fxixj

is positive definite. That is, all the eigenvalues of Hij(x) must be positive for every x. We then define the Legendre transform G(p) as

G(p)=pxF(x)

where

p=F .

Note that

dG=xdp+pdxFdx=xdp ,

which establishes that G is a function of p and that

Gpj=xj .

Note also that the Legendre transformation is self dual, which is to say that the Legendre transform of G(p) is F(x): FGF under successive Legendre transformations.

We can also define a partial Legendre transformation as follows. Consider a function of q variables F(x,y), where x={x1,,xm} and y={y1,,yn}, with q=m+n. Define p={p1,,pm}, and

G(p,y)=pxF(x,y) ,

where

pa=Fxa(a=1,,m) .

These equations are then to be inverted to yield

xa=xa(p,y)=Gpa .

Note that

pa=Fxa(x(p,y),y) .

Thus, from the chain rule,

δab=papb=2Fxaxcxcpb=2Fxaxc2Gpcpb ,

which says

2Gpapb=xapb=K1ab ,

where the m×m partial Hessian is

2Fxaxb=paxb=Kab .

Note that Kab=Kba is symmetric. And with respect to the y coordinates,

2Gyμyν=2Fyμyν=Lμν ,

where

Lμν=2Fyμyν

is the partial Hessian in the y coordinates. Now it is easy to see that if the full q×q Hessian matrix Hij is positive definite, then any submatrix such as Kab or Lμν must also be positive definite. In this case, the partial Legendre transform is convex in {p1,,pm} and concave in {y1,,yn}.


This page titled 2.15: Appendix II- Legendre Transformations is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Daniel Arovas.

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