@article{IM_1968__6_141942,
     author = {F. TAKENS},
     title = {The {Minimal} {Number} of {Critical} {Points} of a {Function} on a {Compact} {Manifold} and the {Lusternik-Schnirelman} {Category.}},
     journal = {Inventiones mathematicae},
     pages = {197--244},
     publisher = {mathdoc},
     volume = {6},
     year = {1968},
     zbl = {0198.56603},
     url = {http://geodesic.mathdoc.fr/item/IM_1968__6_141942/}
}