@article{10_2298_PIM0897001G,
     author = {Ivan Gutman},
     title = {Lower {Bounds} for {Estrada} {Index}},
     journal = {Publications de l'Institut Math\'ematique},
     pages = {1 },
     publisher = {mathdoc},
     volume = {_N_S_83},
     number = {97},
     year = {2008},
     doi = {10.2298/PIM0897001G},
     zbl = {1199.05219},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.2298/PIM0897001G/}
}