# 3.28 FAQ-256 How to write linear constraints in different fitting cases

Last Update: 8/8/2018

In different cases, the ways of writing linear constraints are different.

## Replica fitting

Use parameter name +__n as notation, where n denotes the (n-1)th replica. Note that two underscores are used.

Assume that y0 is a parameter and there is a replica. Then the available notations would be:

y0
y0__2
 Note: the notation is different from that in Origin 7.5.

## Peak Analyzer

The constraint will use parameter name+__n as notation, where n denotes the nth peak. Note that two underscores are used.

Suppose there are two peaks and there are two variables, a and b. The available notations would be:

a__1
b__1
a__2
b__2

A typical example for constraint is  b__1 = b__2.

If there are shared parameters, please use the first parameter in the same group to write the constraint. For example, if parameters a__2, a__3 are shared, the constraint a__1 = a__2 will work while a__1 = a__3 will not work.

 Note: The notation is different from that used in PFM in Origin 7.5. The difference between Peak Analyzer constraint notation and that of replica fitting lies in the first group of parameters. Replica does not use underscore and numeric postfix for the first group of parameter while Peak Analyzer uses.

## Global Fitting

Use parameter name+_n as notation, where n denotes the nth dataset.

Assume that a is a parameter and there are 2 datasets. Then the available notations would be:

a
a_2
 Note: The notation with postfix "_n" would be valid only in global fitting. Independent fit will fit each dataset separately so that "_n" is meaningless.

Keywords:constraint, replica, notation, peak analyzer, parameter, PA