Cylindrical solutions in metric f(R) gravity
  We study static cylindrically symmetric vacuum solutions in Weyl coordinates in the context of the metric f(R) theories of gravity. The set of the modified Einstein equations is reduced to a single equation and it is shown how one can construct exact solutions corresponding to different $f(R)$ models. In particular the family of solutions with constant Ricci scalar ($R=R_{0}$) is found explicitly which, as a special case (R=0), includes the exterior spacetime of a cosmic string. Another new solution with constant, non-zero Ricci scalar is obtained and its possible relation to the Linet-Tian solution in general relativity is discussed. 
A. Azadi, D. Momeni and M. Nouri-Zonoz