This command is used to construct the DOF_Numberer object. The DOF_Numberer object determines the mapping between equation numbers and degrees-of-freedom -- how degrees-of-freedom are numbered.
- Plain -- nodes are assigned degrees-of-freedom arbitrarily, based on the input file. This method is recommended for small problems or when sparse solvers are used, as they do their own internal DOF numbering.
- RCM -- nodes are assigned degrees-of-freedom using the Reverse Cuthill-McKee algorithm. This algorithm optimizes node numbering to reduce bandwidth using a numbering graph. This method will output a warning when the structure is disconnected.
As certain system of equation and solver objects do their own mapping, i.e. SuperLU, UmfPack, Kincho's, specifying a numberer other than plain may not be needed.