Generalized Schl\"omilch's formulas and thermal Casimir effect of a fermionic rectangular box

Schl\"omilch's formula is generalized and applied to the thermal Casimir effect of a fermionic field confined a three-dimensional rectangular box. The analytic expressions of the Casimir energy and Casimir force are derived for arbitrary temperature and edge sizes. The low and high temperature limits and finite temperature cases are considered for the entire parameter space spanned by edge sizes and/or temperature. In the low temperature limit, it is found that for typical rectangular box, the effective 2-dimensional parameter space spanned by the two edge size ratios can be split into four regions according to the nature of the forces. For the waveguide under low temperature, the Casimir force along the longer side of the waveguide cross-section transforms from attractive to repulsive when the aspect ratio of the cross-section exceed a critical value. For the parallel plate scenario under low temperature, our results agrees with previous works. For the finite temperature case, we separate the parameter space into four subcases and various edge size and temperature effects are analyzed. In general, we found that in all cases the Casimir energy is always negative, while the Casimir force at any finite or low temperature can be either repulsive or attractive depending on the sizes of the edges. Finally, for any fixed temperature, there exist a boundary in the parameter space of edge sizes separating the attractive and repulsive regions. Besides, the Casimir energy for an electromagnetic field confined in a three-dimensional box is also derived.