The Fit command has an ‘Arbitrary Fit’ option. This option allows you to specify the form of the function y = f(x) that you want to solve. For detailed documentation see Fit command > Arbitrary Fit.

This article shows you how to save a to a simple function library, so you don’t have to enter from scratch every time.

## Add Function

Consider this example where a points are being fit to normal distribution function.

where σ is the standard deviation and μ is the mean. This is not a standard function under the **Fit function** menu, so select ‘Arbitrary.’ The function can be entered as the following expression:

`1 / (σ * sqrt( 2π )) * exp ( -0.5 (( x - μ ) / σ )^2 )`

To add the function library:

- Enter the function in the
**Function form**entry box. - Click the gear menu to the right of the entry box.
- Select
**Add function**to library.

Now when you create a new **Fit** command, the expression for this function will be listed when you click the same gear menu.

## Remove Function

To remove a function that you entered, that function has to be the current function in the **Function form** entry box. Click the gear menu, and you will now see the option to ‘Remove from function library .’