In \swift, all transformations between thermodynamical quantities are
computed using a pre-defined equation of state (EoS) for the gas. This
allows user to switch EoS without having to modify the equations of
SPH. The definition of the EoS takes the form of simple relations
between thermodynamic quantities. All of them must be specified even
when they default to constants.

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\subsection{Ideal Gas}
\label{sec:eos:ideal}

This is the simplest and most common equation of state. It corresponds
to a gas obeying a pressure law $P = (\gamma-1) \rho u$, where $u$ is
the gas' internal energy, $\rho$ its density and $\gamma$ the
(user defined) adiabatic index, often assumed to be $5/3$ or $7/5$. For such a gas,
we have the following relations between pressure ($P$), internal energy
($u$), density ($\rho$), sound speed ($c$) and entropic function ($A$):

\begin{align}
  P &= P_{\rm eos}(\rho, u) \equiv  (\gamma-1) \rho
  u \label{eq:eos:ideal:P_from_u} \\
  c &= c_{\rm eos}(\rho, u) \equiv \sqrt{\gamma (\gamma-1)
    u} \label{eq:eos:ideal:c_from_u} \\
  A &= A_{\rm eos}(\rho, u) \equiv(\gamma-1) u
  \rho^{\gamma-1} \label{eq:eos:ideal:A_from_u} \\
  ~ \nonumber\\
  P &= P_{\rm eos}(\rho, A) \equiv A
  \rho^{\gamma} \label{eq:eos:ideal:P_from_A} \\
  c &= c_{\rm eos}(\rho, A) \equiv \sqrt{\gamma \rho^{\gamma-1}
    A} \label{eq:eos:ideal:c_from_A} \\
  u &= u_{\rm eos}(\rho, A) \equiv A \rho^{\gamma-1} /
  (\gamma-1) \label{eq:eos:ideal:u_from_A}
\end{align}
Note that highly optimised functions to compute $x^\gamma$ and other
useful powers of $\gamma$ have been implemented in \swift for the most
commonly used values of $\gamma$.


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\subsection{Isothermal Gas}
\label{sec:eos:isothermal}

An isothermal equation of state can be useful for certain test cases
or to speed-up the generation of glass files. This EoS corresponds to
a gas with contant (user-specified) thermal energy $u_{\rm cst}$. For such a gas,
we have the following (trivial) relations between pressure ($P$), internal energy
($u$), density ($\rho$), sound speed ($c$) and entropic function ($A$):

\begin{align}
  P &= P_{\rm eos}(\rho, u) \equiv  (\gamma-1) \rho
  u_{\rm cst} \label{eq:eos:isothermal:P_from_u} \\
  c &= c_{\rm eos}(\rho, u) \equiv \sqrt{\gamma (\gamma-1)
    u_{\rm cst}} = c_{\rm cst} \label{eq:eos:isothermal:c_from_u} \\
  A &= A_{\rm eos}(\rho, u) \equiv(\gamma-1) u_{\rm cst}
  \rho^{\gamma-1} \label{eq:eos:isothermal:A_from_u} \\ 
  ~ \nonumber\\
  P &= P_{\rm eos}(\rho, A) \equiv (\gamma-1) \rho
  u_{\rm cst} \label{eq:eos:isothermal:P_from_A} \\
  c &= c_{\rm eos}(\rho, A) \equiv \sqrt{\gamma (\gamma-1)
    u_{\rm cst}} = c_{\rm cst} \label{eq:eos:isothermal:c_from_A} \\
  u &= u_{\rm eos}(\rho, A) \equiv u_{\rm
    cst} \label{eq:eos:isothermal:u_from_A} 
\end{align}