Partial derivative

Definition at a point

Generic definition

Suppose $f$ is a function of more than one variable, where $x$ is one of the input variables to $f$. Fix a choice $x=x_{0}$ and fix the values of all the other variables. The partial derivative of $f$ with respect to $x$, denoted $\partial f/\partial x$, or $f_{x}$, is defined as the derivative at $x_{0}$ of the function that sends $x$ to $f$ at $x$ for the same fixed choice of the other input variables.

{{#widget:YouTube|id=wPEMBebMm64}}

For a function of two variables

Suppose $f$ is a real-valued function of two variables $x,y$, i.e., the domain of $f$ is a subset of $\mathbb {R} ^{2}$. Suppose $(x_{0},y_{0})$ is a point in the domain of $f$. We define the partial derivatives at $(x_{0},y_{0})$ as follows:

Item	For partial derivative with respect to $x$	For partial derivative with respect to $y$
Notation	${\frac {\partial f(x,y)}{\partial x}}|_{(x,y)=(x_{0},y_{0})}$ Also denoted $f_{x}(x_{0},y_{0})$ or $f_{1}(x_{0},y_{0})$	${\frac {\partial f(x,y)}{\partial y}}|_{(x,y)=(x_{0},y_{0})}$ Also denoted $f_{y}(x_{0},y_{0})$ or $f_{2}(x_{0},y_{0})$
Definition as derivative	${\frac {d}{dx}}f(x,y_{0})|_{x=x_{0}}$. In other words, it is the derivative (at $x=x_{0}$) of the function $x\mapsto f(x,y_{0})$	${\frac {d}{dy}}f(x_{0},y)|_{y=y_{0}}$. In other words, it is the derivative (at $y=y_{0}$) of the function $y\mapsto f(x_{0},y)$.
Definition as limit (using derivative as limit of difference quotient)	$\lim _{x\to x_{0}}{\frac {f(x,y_{0})-f(x_{0},y_{0})}{x-x_{0}}}$ $\lim _{h\to 0}{\frac {f(x_{0}+h,y_{0})-f(x_{0},y_{0})}{h}}$	$\lim _{y\to y_{0}}{\frac {f(x_{0},y)-f(x_{0},y_{0})}{y-y_{0}}}$ $\lim _{h\to 0}{\frac {f(x_{0},y_{0}+h)-f(x_{0},y_{0})}{h}}$
Definition as directional derivative	Directional derivative at $(x_{0},y_{0})$ with respect to a unit vector in the positive $x$-direction.	Directional derivative at $(x_{0},y_{0})$ with respect to a unit vector in the positive $y$-direction.

{{#widget:YouTube|id=vC89LB9mK1g}}{{#widget:YouTube|id=Tj4Nf6xWZh0}}

For a function of multiple variables

The notation here gets a little messy, so read it carefully. We consider a function $f$ of $n$ variables, which we generically denote $(x_{1},x_{2},\dots ,x_{n})$ respectively. Consider a point $(a_{1},a_{2},\dots ,a_{n})$ in the domain of the function. In other words, this is a point where $x_{1}=a_{1},x_{2}=a_{2},\dots ,x_{n}=a_{n}$.

Suppose $i$ is a natural number in the set $\{1,2,3,\dots ,n\}$.

Item	Value for partial derivative with respect to $x_{i}$
Notation	${\frac {\partial }{\partial x_{i}}}f(x_{1},x_{2},\dots ,x_{n})|_{(x_{1},x_{2},\dots ,x_{n})=(a_{1},a_{2},\dots ,a_{n})}$ Also denoted $f_{x_{i}}(a_{1},a_{2},\dots ,a_{n})$ or $f_{i}(a_{1},a_{2},\dots ,a_{n})$
Definition as derivative	${\frac {d}{dx_{i}}}f(a_{1},a_{2},\dots ,a_{i-1},x_{i},a_{i+1},\dots ,a_{n})|_{x_{i}=a_{i}}$. In other words, it is the derivative of the function $x_{i}\mapsto f(a_{1},a_{2},\dots ,a_{i-1},x_{i},a_{i+1},\dots ,a_{n})$ with respect to $x_{i}$, evaluated at the point $x_{i}=a_{i}$.
Definition as a limit (using derivative as limit of difference quotient)	$\lim _{x_{i}\to a_{i}}{\frac {f(a_{1},a_{2},\dots ,a_{i-1},x_{i},a_{i+1},\dots ,a_{n})-f(a_{1},a_{2},\dots ,a_{n})}{x_{i}-a_{i}}}$
Definition as a directional derivative	Directional derivative in the positive $x_{i}$-direction.

{{#widget:YouTube|id=dt-hRS9VNWs}}

Definition as a function

Generic definition

Suppose $f$ is a function of more than one variable, where $x$ is one of the input variables to $f$. The partial derivative of $f$ with respect to $x$, denoted $\partial f/\partial x$, or $f_{x}$ is defined as the function that sends points in the domain of $f$ (including values of all the variables) to the partial derivative with respect to $x$ of $f$ (i.e., the derivative treating the other inputs as constants for the computation of the derivative). In particular, the domain of the partial derivative of $f$ with respect to $x$ is a subset of the domain of $f$.

We can compute the partial derivative of $f$ relative to each of the inputs to $f$.

MORE ON THE WAY THIS DEFINITION OR FACT IS PRESENTED: We first present the version that deals with a specific point (typically with a $\{\}_{0}$ subscript) in the domain of the relevant functions, and then discuss the version that deals with a point that is free to move in the domain, by dropping the subscript. Why do we do this?
The purpose of the specific point version is to emphasize that the point is fixed for the duration of the definition, i.e., it does not move around while we are defining the construct or applying the fact. However, the definition or fact applies not just for a single point but for all points satisfying certain criteria, and thus we can get further interesting perspectives on it by varying the point we are considering. This is the purpose of the second, generic point version.

For a function of two variables

Suppose $f$ is a real-valued function of two variables $x,y$, i.e., the domain of $f$ is a subset of $\mathbb {R} ^{2}$. The partial derivatives of $f$ with respect to $x$ and $y$ are both functions of two variables each of which has domain a subset of the domain of $f$.

Item	For partial derivative with respect to $x$	For partial derivative with respect to $y$
Notation	${\frac {\partial f(x,y)}{\partial x}}$ Also denoted $f_{x}(x,y)$ or $f_{1}(x,y)$	${\frac {\partial f(x,y)}{\partial y}}$ Also denoted $f_{y}(x,y)$ or $f_{2}(x,y)$
Definition as derivative	It is the derivative of the function $x\mapsto f(x,y)$, treating $y$ as an unknown constant	It is the derivative of the function $y\mapsto f(x,y)$, treating $x$ as an unknown constant
Definition as limit (using derivative as limit of difference quotient)	$\lim _{h\to 0}{\frac {f(x+h,y)-f(x,y)}{h}}$	$\lim _{h\to 0}{\frac {f(x,y+h)-f(x,y)}{h}}$
Definition as directional derivative	Directional derivative with respect to a unit vector in the positive $x$-direction.	Directional derivative with respect to a unit vector in the positive $y$-direction.

{{#widget:YouTube|id=fvyCAWWFyKA}}

For a function of multiple variables

Item	Value for partial derivative with respect to $x_{i}$
Notation	${\frac {\partial }{\partial x_{i}}}f(x_{1},x_{2},\dots ,x_{n})$ Also denoted $f_{x_{i}}(x_{1},x_{2},\dots ,x_{n})$ or $f_{i}(x_{1},x_{2},\dots ,x_{n})$
Definition as derivative	It is the derivative of the function $x_{i}\mapsto f(x_{1},x_{2},\dots ,x_{i-1},x_{i},x_{i+1},\dots ,x_{n})$ with respect to $x_{i}$, where all the other variables are treated as unknown constants while doing the differentiation.
Definition as a limit (using derivative as limit of difference quotient)	$\lim _{h\to 0}{\frac {f(x_{1},x_{2},\dots ,x_{i-1},x_{i}+h,x_{i+1},\dots ,x_{n})-f(x_{1},x_{2},\dots ,x_{n})}{h}}$
Definition as a directional derivative	Directional derivative in the positive $x_{i}$-direction.

Related notions

• Derivative
• Higher derivative
• Higher partial derivative

Caveats

Expression for partial derivative depends on all inputs

For further information, refer: Value of partial derivative depends on all inputs

The general expression for the partial derivative with respect to any of the inputs is an expression in terms of all the inputs. For instance, the general expression for $f_{x}(x,y)$ is an expression involving both $x$ and $y$. This is because, even though the $y$-coordinate is kept constant when calculating the partial derivative at a particular point, that constant value need not be the same at all the points.

For instance, consider:

$f(x,y):=x^{2}+y^{2}+xy^{2}$

Then, we have:

$f_{x}(x,y)=2x+y^{2}$

and:

$f_{y}(x,y)=2y+2xy$

Note that each of the expressions involves both the variables $x$ and $y$. In particular, this means that the value of $f_{x}$ at a point depends on both the $x$-coordinate and the $y$-coordinate of the point. Thus, for instance, $f_{x}(2,3)=13$ and $f_{x}(2,4)=20$ are distinct because of the different $y$-values.

In fact, the only cases where the partial derivative with respect to one variable depends only on that variable is where the function is additively separable in terms of a function purely of that variable and a function of the other variables.

Meaning of partial derivative depends on all variables

For further information, refer: Meaning of partial derivative depends on entire coordinate system

This is a very subtle but very important point. It says that the partial derivative with respect to one variable depends not only on the choice of that particular variable, but on the choice of the other variables that are being kept constant for the purpose of computing the partial derivative. If a coordinate transformation is performed that changes what those other variables are, that could affect the value of the partial derivative.

This has a very real-world corollary. In economics and social science, we often talk of the partial derivative with respect to one variable as measuring what happens ceteris paribus on the other variables. However, the notion of ceteris paribus on other variables depends on what the other variables are. If we redefine the coordinate system to change that meaning, the partial derivative can change.

For instance, consider the function:

$u=f(x,y):=2x+3y$

In this case, we have:

${\frac {\partial u}{\partial x}}=f_{x}(x,y)=2$

Now, suppose we consider $f$ in terms of $x$ and $v=x+y$. Then, $f(x,y)$, as a function of $x$ and $v$, is $3v-x$. Thus, we get:

${\frac {\partial u}{\partial x}}=-1$

Note that the two partial derivatives with respect to $x$ are not equal. The reason for this is that in the first case, we are taking the partial derivative with respect to $x$ keeping $y$ constant, whereas in the second case, we are taking the partial derivative with respect to $x$ keeping $v=x+y$ constant.