In fitteia consider the following definitions in the appropriate fields in the fitter module:

1. Independent variable: tau

2. Dependent variable: Mz

3. Expression: Mz=Minf\+(M0-Minf)*1.0/(1+p21+p31)*exp(-tau/T11)\+(M0-Minf)*p21/(1+p21+p31)*exp(-tau/T12))\+(M0-Minf)*p31/(1+p21+p31)*exp(-tau/T13)

4. fitting parameters: Minf,M0,p21,p31,T11,T12,T13

The "\+" will produce a plot where "Mz" and the contributions "Minf", "(M0-Minf)*1.0/(1+p21+p31)*exp(-tau/T11)", "(M0-Minf)*p21/(1+p21+p31)*exp(-tau/T12))", and "(M0-Minf)*p31/(1+p21+p31)*exp(-tau/T13)" will be drawn.

In other fitting software substitute "\+" by "+".