Thi Brouwer's invarianceof domainIf fi R IR is a continuous injection then it is open

p.rosketch

It suffices to show that for any continuous injection f D R

flo E tut f DFirst note that f D fLD is closed since a closed

set AED is compact mapped to a compact set FIAwhich is closed since it is a compact subset of a Hausdorff spaceSo f i f ID D is continuous

Then it also has a cont extension G IR IR

Tietze extension then for which G fa 0

Assume flo E JD i e the zero of G lies on the boundary of f DThen we can construct a perturbation c C f ID IR with HÜ Ellothat has no zwo in f DHowever by Brouwer'sfixedpoint tun D fw G G fln

must have a freed point 2 ED for which then FG 0 I

Co invariance of dimension There is a cont injection fi R IR

iffnrm.proLet nein e R IR 40 Suppose f IR R is a cont injection

Then LofiR R wouldbe a cont injection that however cannot be open D

IT EMBEDDINGS

Rec A smooth map fM N between smooth manifolds is an

immersion if txc.tl rank df dim M and it is a smooth

embedding if it is an immersion fi Ms fLM a homeomorphism

Equivalently it is a smoothembedding if f Msflm is a diffeomorphism

Lemmai If Me is compact auf injective immersion is a smooth embedding

pro.fi It remains to show that f f IN M is continuous which is

equivalent to fin fLM beingclosed

Let AEM be closed Since M is compact A is compact as well

and so is f A by continuity of f Being a compact subset

of a Hausdorff space FIA is closed D

Thm.ie Let M be a smooth m manifold fitt M an injective immersion and

Pv IR 71 D theorthogonal projection onto the N 1 dim

subspaceorthogonal to r ES i e 71 RR Irre R xiv

If Ns 2Mt 1 then for all VES except a setof measure zwo in SProf M 71 IR is an injective immersion

proo.fi We first prove infectivity To this end define a smooth map

g Mx MI IM S by g yt Since In flat CMeint

Kfw fullp is closed Mehl b is a smooth Zen manifold

The image of g has measure zero in S if Zu N I as assumed

However if Prof is not injective then there are xtys.t.fm fly ar

so necessarily v Eg Mehl In

To prove the immersion property we argue similarly define

h TMI Mxto S as h x id where EM andHdfxill

EIN Ito Again the image of h has measure zero if N 1 Zu

For Prof to be an immersion we have to have rank DLRf m

for all EM However DIRf Rdf has rank m for all EMunless v is in the image of h B

Caroline Whitney's embedding then easyversion

If M is a compact manifold of dim m then it can be embedded

in

RZmtt.proBy a smooth versionof the embedding then there is a smoothembedding into some R Due to compactness injective immersions are

embeddings Iterating the previous thin we can reduce the dimension

down to 2Mt 1 D

reina We can learn two more things from theproof

embeddings are not exceptional and in fact dense in the set of allsmooth maps into R see Hirsch for a proof

if we are only interested in an immersion we can reduce the

dimension further by one via restricting to unit vectors in the tangent

space

Thm Whitney's embedding immersion theorem strongversion Any smooth

m manifold can be smoothly embedded in R and immersed in R if m 1

remart this is the best possible affine bound since RP cannotbe

embedded in IR if u is a powerof two

for immersions a tight bound is known provenby Cohen in 85

any compactsmooth m manifold can be immersed in R

where alm is the number of ones in the binary expansion of m