# Conditions for Function to be Maximum of its Legendre Transform Two-variable Equivalent

## Theorem

Let $x, p \in \R$.

Let $\map f x$ be a strictly convex real function.

Let $f^*$ be a Legendre transformed $f$.

Let $\map g {x, p} = - \map {f^*} p + x p$

Then:

- $\displaystyle \map f x = \max_p \paren{- \map {f^*} p + x p}$

where $\displaystyle \max_p$ maximises the function with respect to a variable $p$.

## Proof

Function $g$ acquires an extremum along $p$, when its first derivative along $p$ vanishes:

\(\displaystyle \frac {\partial g} {\partial p}\) | \(=\) | \(\displaystyle -\frac{\partial f^*} {\partial p} + x\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle 0\) | Extremum condition | ||||||||||

\(\displaystyle \leadsto \ \ \) | \(\displaystyle \map { {f^*}'} p\) | \(=\) | \(\displaystyle x\) |

To check if the extremum is a global maximum, consider the second derivative:

- $\dfrac {\partial^2 g} {\partial p^2} = - \dfrac {\partial^2 f^*} {\partial p^2}$

By Convexity of Function implies Convexity of its Legendre Transform and Real Function is Strictly Convex iff Derivative is Strictly Increasing

it holds that

\(\displaystyle \frac {\partial^2 f^*} {\partial p^2}\) | \(>\) | \(\displaystyle 0\) | |||||||||||

\(\displaystyle \leadsto \ \ \) | \(\displaystyle \frac {\partial^2 g} {\partial p^2}\) | \(<\) | \(\displaystyle 0\) | ||||||||||

\(\displaystyle \leadsto \ \ \) | \(\displaystyle \frac {\partial^2 g} {\partial p^2} \bigg \rvert_{\frac {\partial g} {\partial p} \mathop = 0}\) | \(<\) | \(\displaystyle 0\) |

Negative second derivative at the extremum implies extremum being a global maximum.

Therefore:

- $\displaystyle \max_p \paren{- \map {f^*} p + x p} = \paren{- \map {f^*} p + x p} \big \rvert_{x = \map {{f^*}'} p}$

The right hand side is the Legendre Transform of $f^*$.

However, $f^*$ is a Legendre transformed $f$.

By Legendre Transform is Involution, this equals $f$.

$\blacksquare$

## Sources

- 1963: I.M. Gelfand and S.V. Fomin:
*Calculus of Variations*... (previous) ... (next): $\S 4.18$: The Legendre Tranformation