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 x₀ in ln(x/x₀) 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: