# extended forms from foliations [closed]

hi,

i have the following question: Let $M$ be a n-dimensional manifold (or riemannian or everything thats nice ...) and let $\mathcal{F}$ be a foliation of $M$ by surfaces. Assume, furthermore, that on each leaf $\mathcal{L}$ there is a two form $\alpha_{\mathcal{L}}$. is it possible to construct a globally defined (smooth) 2-form $\alpha$ on $M$ out of the $\alpha_{\mathcal{L}}$? or are there any good books or papers were this is discussed? thanks in advance.

janson

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## closed as too localized by Ryan Budney, Deane Yang, Willie Wong, Alain Valette, Mark SapirDec 5 '11 at 6:37

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You appear to have misspelled your own name. As another person named "Jason", this has happened to me as well. – Jason Starr Dec 3 '11 at 13:55
upps sorry ... actually its janson :). – gary Dec 3 '11 at 13:57
What do you mean by $\alpha$ is defined on the foliation -- which topology on the foliation are you using -- is there any sense in which $\alpha$ is continuous or differentiable? Perhaps give a concrete example using an irrational foliation of the torus to let us know what context you're using. Regardless you question seems likely best-suited for math.stackexchange.com. – Ryan Budney Dec 3 '11 at 20:35

When your manifold is Riemanniann, you can do something obvious: compose $\alpha_{\mathbb{L}}$ with the orthogonal projection from $T_x M$ to $T_x \mathcal{L}$ when $x\in \mathcal{L}$. But you will need some transverse regularity for the $\alpha_{\mathcal{L}}$ to ensure the resulting form to be smooth. This is not really a restriction, since anyway you can easily see that there are families of $\alpha_{\mathcal{L}}$ such that no $2$-forms $\alpha$ on $M$ can be smooth and restrict to $\alpha_{\mathcal{L}}$ along each leaf.
Edit: About the "transverse regularity": if you compute the form $\alpha$ I propose, you'll see that it need not be smooth (or even continuous !); you need to assume that $\alpha_{\mathcal{L}_x}$ (where $\mathcal{L}_x$ is the leaf through $x$), depends smoothly on $x$. This is stronger than asking that each $\alpha_{\mathcal{L}}$ is smooth along $\mathcal{L}$; the difference is mainly about transverse variation of the base point.
what do you mean by a transverse regularity for the $\alpha_{\mathcal{L}}$ ? – gary Dec 3 '11 at 14:20
if i had this regularity, does this ensure the smootness of $\alpha$? – gary Dec 3 '11 at 14:23