nLab transversal maps




Two differentiable functions f:XZf \colon X \to Z and g:YZg \colon Y \to Z between differentiable manifolds are called transversal if, roughly, the images of XX and YY in ZZ do not “touch tangentially”.

If ff or gg are inclusions of (possibly immersed) submanifolds one speaks of these submanifolds being transversal to each other.


Two differentiable functions f:XZf \colon X \to Z and g:YZg \colon Y \to Z between differentiable manifold (for instance smooth functions between smooth manifolds) are transversal if for all pairs of points xXx \in X and yYy \in Y with f(x)=z=g(y)f(x) = z = g(y) the differentials of ff and gg in these points span the entire tangent space at zz in the sense that

im(df)+im(dg)T zZ. im(d f) + im(d g) \simeq T_z Z \,.

(Notice that this is not required to be a direct sum.)



Pullbacks along transversal maps

Various constructions involving pullbacks of differentiable manifolds work as expected only for pullbacks involving transversal maps.

For example, two differentiable functions with a common codomain are transversal only if their pullback exists and is preserved by the tangent bundle functor; that is, T(X× ZY)=TX× TZTYT(X \times_Z Y) = T X \times_{T Z} T Y. (However, the pullback may exist and be preserved without transversality; for example if XX and YY are both abstract points, ZZ is not a point, and the maps X,YZX, Y \to Z are equal as concrete points of ZZ.)

This is to be regarded as the dual of the possibly more familiar statement that various constructions involving quotients only work as expected for free actions.

Both of these “problems” are solved by passing from the ordinary 1-category Diff of manifolds to a suitable higher category of generalized smooth spaces.

More precisely:

Thom’s transversality theorem

See at Thom's transversality theorem.


Last revised on January 20, 2024 at 17:37:33. See the history of this page for a list of all contributions to it.