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## Multivariable calculus

### Unit 1: Lesson 6

Visualizing multivariable functions (articles)

# Parametric functions, one parameter

Parametric functions give a way to represent functions with a one-dimensional input and a multidimensional output.

## What we're building to

• A function with a one-dimensional input and a multidimensional output can be thought of as drawing a curve in space.
• Such a function is called a parametric function, and its input is called a parameter.
• Sometimes in multivariable calculus, you need to find a parametric function that draws a particular curve. This is called parameterizing that curve.

## Visualizing vector-valued functions

So you're off, happily reading some math one day, as one does, and you come across a function like this:
$\displaystyle f(t) = \left[ \begin{array}{c} t \cdot \cos(2\pi t) \\ t \cdot \sin(2\pi t) \end{array} \right]$
How would you visualize it?
It takes in a single variable, t, and outputs a two-dimensional vector. For example, at the input t, equals, 1, it is evaluated like this.
$\displaystyle f(1) = \left[ \begin{array}{c} 1 \cdot \cos(2\pi \cdot1) \\ 1 \cdot \sin(2\pi \cdot 1) \end{array} \right]= \left[ \begin{array}{c} 1 \\ 0 \end{array} \right]$
This output is a vector with length 1 pointing in the x-direction.
But how do you visualize all outputs all at once?
A nice way to do this is to imagine what curve the tip of that vector will trace out as t ranges over some values. For example, the following interactive diagram lets you see what curve the output of f will trace out as t ranges from 0 to 3:
This is called a parametric curve. When you choose to interpret the function this way, it is called a parametric function, and the input t is called the parameter.

## Only look at output space

Notice, unlike graphs, where we try to depict both the input space and output space of a function at the same time, or contour maps, where we just draw on the input space, interpreting functions parametrically has us looking purely at the output space. The reason this makes sense for the example above is that the output space has more dimensions than the input space.

## Input information is lost

The problem with only drawing in the output space is that it is not immediately clear which inputs went to the output values we draw. For instance, consider these two functions:
\begin{aligned} \blueE{f(t)} &= \left[ \begin{array}{c} \cos(t) \\ \sin(t) \end{array} \right] \\\\ \redE{g(t)} &= \left[ \begin{array}{c} \cos(t+\pi) \\ \sin(t+\pi) \end{array} \right] \end{aligned}
If we plot these as parametric functions, with t running from 0 to 2, pi, they each draw a circle with radius 1 centered at the origin.
Circle
However, they are different functions. For instance, evaluate each one at t, equals, 0.
Given that $\blueE{f(t)} = \left[\begin{array}{c} \cos(t) \\ \sin(t) \end{array} \right]$, what is start color #0c7f99, f, left parenthesis, 0, right parenthesis, end color #0c7f99?

Given that $\redE{g(t)} = \left[\begin{array}{c} \cos(t+\pi) \\ \sin(t+\pi) \end{array} \right]$, what is start color #bc2612, g, left parenthesis, 0, right parenthesis, end color #bc2612?

One way is to keep track of the lost input information is to label a few points with the input values
$\blueE{f(t)} = \left[\begin{array}{c} \cos(t) \\ \sin(t) \end{array} \right]$
First parameterization of circle
$\redE{g(t)} = \left[\begin{array}{c} \cos(t+\pi) \\ \sin(t+\pi) \end{array} \right]$
Second parameterization of circle
Alternatively, you could imagine how the curve is drawn over time as t runs from the start value to the end value. This is particularly relevant when the function is meant to model the trajectory of a particle through space.

## Parametrization

In multivariable calculus, and especially in a topic called "line integration", it is common to start off with a curve, and search for a parametric function that draws the curve. One example that comes up a lot is the unit circle, meaning the circle with radius 1 centered at the origin.
Circle
Finding a parametric function that describes a curve is called parameterizing that curve. In the previous section I showed two different functions which parameterize the unit circle. The most common one that people use in practice is this one:
$f(t) = \left[\begin{array}{c} \cos(t) \\ \sin(t) \end{array} \right]$
Note: When you are parameterizing a curve, you must not only specify the parametric function, but also the range of input values that will draw the curve. For example, using the function f, left parenthesis, t, right parenthesis to draw the unit circle above, you could let t range from 0 to 2, pi.

## Example: Parameterizing a loopy curve

Let's say you want to parameterize this loopy pattern:
Draw circle while being pushed right
To parameterize a curve, you should always think about drawing it. In this case, you could imagine sketching it by trying to draw a circle counterclockwise while someone pushes your hand to the right at a steady velocity. To encode this, using formulas, we start with parametric function for a circle:
$\displaystyle f(t) = \left[ \begin{array}{c} \cos(t) \\ \sin(t) \end{array} \right]$
This would have us starting at the point left parenthesis, 1, comma, 0, right parenthesis, and tracing a circle with radius 1 counterclockwise. Since the loopy curve we are parameterizing starts at left parenthesis, minus, 2, comma, 0, right parenthesis, we start tweaking this function by shifting the x value by minus, 3.
$\displaystyle f(t) = \left[ \begin{array}{c} \cos(t) -3 \\ \sin(t) \end{array} \right]$
Being pushed to the right over time corresponds with a steady increase in the x-value of your hand with respect to time, unrelated to the motions it's going through for the circle. To encode this, add some constant start color #bc2612, c, end color #bc2612 times t to the x-component of the function.
$\displaystyle f(t) = \left[ \begin{array}{c} \cos(t) -3 + \redE{c}t\\ \sin(t) \end{array} \right]$
To figure out what the constant should be, we need to know how far right one has moved after completing one loop. Our current function f, left parenthesis, t, right parenthesis completes one loop as t goes from 0 to 2, pi. Looking at the loopy curve, it seems we shift exactly 1 to the right after a single loop.
Distance one has traveled to the right after one loop
This means we must have 2, pi, start color #bc2612, c, end color #bc2612, equals, 1, and hence start color #bc2612, c, equals, start fraction, 1, divided by, 2, pi, end fraction, end color #bc2612.
$\displaystyle f(t) = \left[ \begin{array}{c} \cos(t) -3 + \redE{\frac{1}{2\pi}}t\\ \sin(t) \end{array} \right]$
Finally, we need to place bounds on t. Let's see just how many loops the loopy curve contains:
Draw circle while being pushed right
It looks like it has 6. Since our chosen function f, left parenthesis, t, right parenthesis completes one loop as t increases by 2, pi, we should let it range from 0 to 6, left parenthesis, 2, pi, right parenthesis, equals, 12, pi.

## Summary

• A function with a one-dimensional input and a multidimensional output can be thought of as drawing a curve in space.
• Such a function is called a parametric function, and its input is called a parameter.
• Sometimes in multivariable calculus, you need to find a parametric function that draws a particular curve. This is called parameterizing that curve.