Update README.md

README.md

@@ -195,7 +195,7 @@ c) the sign of the resulting stretch from a small superposed force in one direct
 For tensor-valued material definitions use `MaterialTensor` (e.g. any stress-strain relation). Also, please have a look at [casADi's documentation](https://web.casadi.org/). It is very powerful but unfortunately does not support all the Python stuff you would expect. For example Python's default if-else-statements can't be used in combination with symbolic conditions (use `math.if_else(cond, if_true, if_false)` instead). Contrary to [casADi](https://web.casadi.org/), the gradient of the eigenvalue function is stabilized by a perturbation of the diagonal components.
 
 ### Example for a Neo-Hookean material model as (u/p)-formulation
-The (u/p)-formulation is created by an instance of `MaterialTensor`. If the argument `triu` is set to `True` the gradient method returns only the upper triangle entries of the gradient components. If some of the input variables are internal state variables the number of these variables have to be passed to the optional argument `statevars`. While the hyperelastic material classes are defined by a strain energy function, this one is defined by the first Piola-Kirchhoff stress tensor. State variables are the same as default variables but they are excluded from gradient calculations. They may also be used for additional quantities, e.g. the initial deformation gradient at the beginning of an increment or the time increment. Hence, it is a very flexible class not restricted to hyperelasticity.
+The (u/p)-formulation is created by an instance of `MaterialTensor`. If the argument `triu` is set to `True` the gradient method returns only the upper triangle entries of the gradient components. If some of the input variables are internal state variables the number of these variables have to be passed to the optional argument `statevars`. While the hyperelastic material classes are defined by a strain energy function, this one is defined by the first Piola-Kirchhoff stress tensor. Internally, state variables are equal to default variables but they are excluded from gradient calculations. State variables may also be used as placeholders for additional quantities, e.g. the initial deformation gradient at the beginning of an increment or the time increment. Hence, it is a very flexible class not restricted to hyperelasticity.
 
 ```python
 from matadi import MaterialTensor, Variable
@@ -233,6 +233,8 @@ dWdF, dWdp, statevars_new = NH.function([defgrad, pressure, statevars])
 d2WdFdF, d2WdFdp, d2Wdpdp = NH.gradient([defgrad, pressure, statevars])
 ```
 
+**Hint**: *The state variable concept is also implemented for the `Material` class.*
+
 Simple examples for using `matadi` with [`scikit-fem`](https://github.com/adtzlr/matadi/discussions/14#) as well as with [`felupe`](https://github.com/adtzlr/matadi/discussions/22) are shown in the Discussion section.
 
 ## References