@article{CC_2005_34_3_a4,
     author = {Clarke, Francis},
     title = {The maximum principle in optimal control, then and now},
     journal = {Control and Cybernetics},
     pages = {709--722},
     publisher = {mathdoc},
     volume = {34},
     number = {3},
     year = {2005},
     zbl = {1167.49311},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CC_2005_34_3_a4/}
}