Editing Neutron transport (section)

===Discretization in deterministic methods===
To numerically solve the transport equation using algebraic equations on a computer, the spatial, angular, energy, and time variables must be [[Discretization|discretized]]. 
* Spatial variables are typically discretized by simply breaking the geometry into many small regions on a mesh. The balance can then be solved at each mesh point using [[finite difference]] or by nodal methods. 
* Angular variables can be discretized by discrete ordinates and weighting [[Numerical integration|quadrature sets]] (giving rise to the [[Discrete Ordinates Method|S<sub>N</sub> methods]]), or by functional expansion methods with the [[spherical harmonics]] (leading to the P<sub>N</sub> methods).
* Energy variables are typically discretized by the multi-group method, where each energy group represents one constant energy. As few as 2 groups can be sufficient for some [[Thermal-neutron reactor|thermal reactor]] problems, but [[Fast-neutron reactor|fast reactor]] calculations may require many more. 
* The time variable is broken into discrete time steps, with time derivatives replaced with difference formulas.