Wormholes in viable $f(R)$ modified theories of gravity and Weak Energy   Condition
  In this work wormholes in viable $f(R)$ gravity models are analysed. We are interested in exact solutions for stress-energy tensor components depending on different shape and redshift functions. Several solutions of gravitational equations for different $f(R)$ models are examined. Found solutions imply no need for exotic material, while this need is implied in the standard general theory of relativity. Simple expression for WEC violation near the throat is derived and analysed. High curvature regime is also discussed, as well as the question of the highest possible values of Ricci scalar for which WEC is not violated near the throat, and corresponding functions are calculated for the several models. The approach here differs from the one that has been common since no additional assumptions to simplify the equations are made, and functions in $f(R)$ models are not taken to be arbitrary functions, but rather a feature of the theory that has to be evaluated on the basis of consistency with observations for the Solar System and cosmological evolution. 
Petar Pavlovi\'c and Marko Sossich
gr-qc