Eigen defines several typedef shortcuts for most common 1D and 2D array types.
The general patterns are the following:
ArrayRowsColsType where Rows and Cols can be 2,3,4 for fixed size square matrices or X for dynamic size, and where Type can be i for integer, f for float, d for double, cf for complex float, cd for complex double.
For example, Array33d is a fixed-size 3x3 array type of doubles, and ArrayXXf is a dynamic-size matrix of floats.
There are also ArraySizeType which are self-explanatory. For example, Array4cf is a fixed-size 1D array of 4 complex floats.