## Curve Fitting with Constraints

**Use constraints** on fit coefficients to express physical requirements,
such as requiring intensity peaks in a spectrum to have positive amplitude.

**Use constraints** to prevent fit coefficients from taking on undefined values,
like requiring x_{0} in ln(x/x_{0}) to be positive.
A constraint like this can mean the difference
between success and failure in curve fitting.

Igor Pro allows you to specify **minimum** and **maximum** values for each fit coefficient or for
**linear combinations** of fit coefficients.

Simple constraints on the value of a single fit coefficient can be entered directly in the Curve Fitting dialog:

More complex constraints are expressed as arithmetic expressions in a string array which can be edited in a table: