The **GSS library** (**G**eneralized **S**tate **S**pace) is the core of the SMAC Toolbox. **It implements a new Matlab class, which allows to model uncertain and nonlinear systems as Linear Fractional Representations**. It replaces and extends the **LFR Toolbox**, with a more intuitive way to describe LFR and a more user-friendly interface, including a Simulink library. Several tools are proposed to manipulate GSS objects (addition, multiplication, inversion, concatenation, feedback...), obtain GSS objects from symbolic models, convert GSS/LFR/USS objects, manipulate the uncertainties and the nonlinearities (normalization, reordering, random sampling), and perform order reduction or approximation. A large class of continuous- and discrete-time systems can be handled, with real or complex uncertain or varying parameters, polytopic-type uncertain or varying elements, linear time-invariant uncertainties, sector nonlinearities, saturations, deadzones, or more general nonlinear operators. Full compatibility is also ensured with other modeling, analysis and control libraries of the SMAC toolbox (**APRICOT**, **SMART**, **IQC**, **SAW**...).

The **APRICOT library** (**A**pproximation of **P**olynomial and **R**ational-type for **I**ndeterminate **C**oefficients via **O**ptimization **T**ools) is an optional module, which is compatible with both the GSS library and the LFR toolbox. **It allows to convert a set of scalar, vector or matrix samples into sparse polynomial or rational expressions, for which the number of terms in the numerator and denominator is as low as possible**. Simple yet accurate LFR are obtained, which are tractable both for analysis and design purposes.

Note that the **LFR toolbox** (**L**inear **F**ractional **R**epresentation) is not maintained anymore since 2014, but its latest version can still be downloaded.