Conformal killing vectors of plane symmetric four dimensional lorentzian manifolds Suhail Khan, Tahir Hussain, Ashfaque H. Bokhari and Gulzar Ali Khan Eur. Phys. J. C, 75 11 (2015) 523 Published online: 03 November 2015 DOI: 10.1140/epjc/s10052-015-3758-1