# Preface

## Discussion Forum

We’re always happy to help out with {feelpp} or any other questions you might have. You can ask a question or signal an issue at the Gitter {feelpp} salon.

Join the {feelpp} chat

This book is available on Github. We use Gitter to discuss the changes in the book.

Join the {feelpp} book chat

## Conventions used in this book

The following typographical conventions are used in the book

Italic indicates new terms

`typewriter` is used on program listings as well as when referring to programming elements, e.g. functions, variables, statements, data types, environment variables or keywords.

` typewriter` or `> typewriter` displays commands that the user types literally without the `` or `>`.

 this is a general note.
 this is a general warning.
 be cautious

## Mathematical Notations

### Geometry and Meshes

• d=1,2,3 geometrical dimension

• \Omega \subset \mathbb{R}^d

• K a cell or element of a mesh

• h characteristic mesh size

• k_{\mathrm{geo}} polynomial order of the geometrical transformation

• \delta=(h,k_{\mathrm{geo}}) discretization parameter pair for the geometrical transformation, default value k_{\mathrm{geo}}=1 (straight cells or elements)

• \varphi^K_\delta: \hat{K} \rightarrow K, geometrical transformation

• \mathcal{T}_{\delta} a triangulation, \mathcal{T}_\delta = \{ K\; | \; K=\varphi^K_\delta (\hat{K}) \}

• \Omega_h \equiv \cup_K {K}

### Spaces

• P^k_{c,h} = \{ v_h \in C^0(\bar{\Omega}); \forall K \in \mathcal{T}_h,\ v_h \circ T_K \in \mathbb{P}^k\} Space of continuous piecewise polynomial of total degree \leq k.