@article{PIM_1967_N_S_07_21_a20,
     author = {M. D. Leko},
     title = {About twin paradox},
     journal = {Publications de l'Institut Math\'ematique},
     pages = {205 },
     publisher = {mathdoc},
     volume = {_N_S_07},
     number = {21},
     year = {1967},
     url = {http://geodesic.mathdoc.fr/item/PIM_1967_N_S_07_21_a20/}
}