# Definition:Partial Derivative/Second Derivative

## Definition

Let $\map f {x, y}$ be a function of the two independent variables $x$ and $y$.

The second partial derivatives of $f$ with respect to $x$ and $y$ are defined and denoted by:

 $(1):\quad$ $\displaystyle \dfrac {\partial^2 f} {\partial x^2}$ $=$ $\displaystyle \map {\dfrac \partial {\partial x} } {\dfrac {\partial f} {\partial x} }$ $\displaystyle =: \map {f_{1 1} } {x, y}$ $(2):\quad$ $\displaystyle \dfrac {\partial^2 f} {\partial y^2}$ $=$ $\displaystyle \map {\dfrac \partial {\partial y} } {\dfrac {\partial f} {\partial y} }$ $\displaystyle =: \map {f_{2 2} } {x, y}$ $(3):\quad$ $\displaystyle \quad \dfrac {\partial^2 f} {\partial x \partial y}$ $=$ $\displaystyle \map {\dfrac \partial {\partial x} } {\dfrac {\partial f} {\partial y} }$ $\displaystyle =: \map {f_{2 1} } {x, y}$ $(4):\quad$ $\displaystyle \dfrac {\partial^2 f} {\partial y \partial x}$ $=$ $\displaystyle \map {\dfrac \partial {\partial y} } {\dfrac {\partial f} {\partial x} }$ $\displaystyle =: \map {f_{1 2} } {x, y}$

## Examples

### Example: $u + \ln u = x y$

Let $u + \ln u = x y$ be an implicit function.

Then:

$\dfrac {\partial^2 u} {\partial y \partial x} = \dfrac {\partial^2 u} {\partial x \partial y} = \dfrac u {u + 1} + \dfrac {x y u} {\paren {u + 1}^2}$