Let (F) be a foliation of codimension 2 on a compact manifold with at least one non-compact leaf. We show that then (F) must contain uncountably many non-compact leaves. We prove the same statement for oriented p–dimensional foliations of arbitrary codimension if there exists a closed p form which evaluates positively on every compact leaf. For foliations of codimension 1 on compact manifolds it is known that the union of all non-compact leaves is an open set [A Haefliger, Varietes feuilletes, Ann. Scuola Norm. Sup. Pisa 16 (1962) 367–397].
Vogt, Elmar  1
@article{10_2140_agt_2002_2_257,
author = {Vogt, Elmar},
title = {Foliations with few non-compact leaves},
journal = {Algebraic and Geometric Topology},
pages = {257--284},
year = {2002},
volume = {2},
number = {1},
doi = {10.2140/agt.2002.2.257},
url = {http://geodesic.mathdoc.fr/articles/10.2140/agt.2002.2.257/}
}
Vogt, Elmar. Foliations with few non-compact leaves. Algebraic and Geometric Topology, Tome 2 (2002) no. 1, pp. 257-284. doi: 10.2140/agt.2002.2.257
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