Casimir effect with a helix torus boundary condition
  We use the generalized Chowla-Selberg formula to consider the Casimir effect of a scalar field with a helix torus boundary condition in the flat ($D+1$)-dimensional spacetime.   We obtain the exact results of the Casimir energy density and pressure for any $D$ for both massless and massive scalar fields. The numerical calculation indicates that once the topology of spacetime is fixed, the ratio of the sizes of the helix will be a decisive factor. There is a critical value $r_{crit}$ of the ratio $r$ of the lengths at which the pressure vanishes. The pressure changes from negative to positive as the ratio $r$ passes through $r_{crit}$ increasingly. In the massive case, we find the pressure tends to the result of massless field when the mass approaches zero. Furthermore, there is another critical ratio of the lengths $r_{crit}^{\prime}$ and the pressure is independent of the mass at $r=r_{crit}^{\prime}$ in the D=3 case. 
Xiang-hua Zhai, Xin-zhou Li and Chao-Jun Feng
hep-th