Let $M$ be a (real) manifold. Recall that an *analytic structure* on $M$ is an atlas such that all transition maps are real-analytic (and maximal with respect to this property). (There's also a sheafy definition.) So in particular being analytic is a structure, not a property.


Q1: Is it true that any topological manifold can be equipped with an analytic structure?


Q2: Can any $C^\infty$ manifold (replace "analytic" by "$C^\infty$" in the first paragraph) be equipped with an analytic structure (consistent with the smooth structure)?



Can every manifold be given an analytic structure?
(similar to Mariano's post)


Q1: no. There are topological manifolds that don't admit triangulations, let alone smooth structures. All smooth manifolds admit triangulations, this is a theorem of Whitehead's. The lowest-dimensional examples of topological manifolds that don't admit triangulations are in dimension 4, the obstruction is called the Kirby-Siebenmann smoothing obstruction.


Q2: $C^1$ manifolds all admit compatible $C^\infty$ and analytic ($C^\omega$) structures. This is a theorem of Hassler Whitney's, in his early papers on manifold theory, where he proves they embed in euclidean space. The basic idea is that your manifold is locally cut out of euclidean space by $C^1$-functions so you apply a smoothing operator to the function and then argue that the level-set does not change (up to $C^1$-diffeomorphism), provided your smoothing approximation is small enough in the $C^1$-sense. I'm not sure who gets the original credit but you can go much further -- compact boundaryless smooth manifolds are all realizable as components of real affine algebraic varieties, planar linkages in particular. There's a Millson and Kapovich paper, *[Universality theorems for configuration spaces of planar linkages](https://arxiv.org/abs/math/9803150)* on the topic available. It seems people give a lot of credit to Bill Thurston.


*edit: Some time ago Riccardo Benedetti sent me some comments to append to my answer. They appear below, with some minor MO-formatting on my part.*


In the famous paper "Real algebraic manifolds" (Annals of Math 56, 3, 1952), John Nash just proved that:


"Every compact closed smooth manifold M embedded in some $R^N$, with $N$ big enough (as usually $N=2Dim(M)+1$ suffices), can be smoothly approximated by a union of components, say $M\_a$, of the non-singular locus of a real algebraic subset $X$ of $R^N$."


In the same paper he stated also some conjectures/questions, in particular whether one can get $M\_a = X$ (so that $X$ is a non-singular real algebraic model of $M$), or whether one can even get such an algebraic model which is rational.


A.H. Wallace (1957) solved positively the first question under the assumption that $M$ is a boundary. Finally a complete positive answer was given by A. Tognoli (1973) by using, among other things, a so called "Wallace trick" and the fact (due to Milnor) that the smooth/un-oriented bordism group is generated by real algebraic projective non singular varieties.


Starting from this Nash-Tognoli theorem, mostly in the 80's-90's of the last century, a huge activity has been developed about the existence of real algebraic models for several instances of smooth or polyhedral structures, with major contributions by S. Akbulut and H. King (in particular they proved that if M is embedded in $R^n$, an algebraic model can be realized in $R^{n+1}$; to my knowledge it is open if we can stay in the given $R^n$).


If I am not wrong, the realization of real algebraic varieties via planar linkages (with related credit to Thurston) does not provide an alternative proof of Nash-Tognoli theorem.


The "Nash rationality conjecture" is more intriguing and has been basically "solved" in dimension less or equal to 3. This is mentioned for instance in some answers to the questions:


[What's the difference between a real manifold and a smooth variety?](https://mathoverflow.net/questions/23911/whats-the-difference-between-a-real-manifold-and-a-smooth-variety/23944#23944)


"You might also be interested in some of the articles by Kolla'r on the Nash conjecture contrasting real varieties and real manifolds. such as
"What are the simplest varieties?", Bulletin, vol 38. I like the pair of theorems 54, 51, subtitled respectively: "the Nash conjecture is true in dim 3",
and "The Nash conjecture is false in dim 3".


[What is known about the MMP over non-algebraically closed fields](https://mathoverflow.net/questions/23107/what-is-known-about-the-mmp-over-non-algebraically-closed-fields/41737#41737)


"Another issue is the rational connectivity and its relation to Mori fiber spaces ...... To illustrate the difficulties, here is a conjecture of Nash (yes, that Nash):


Let $Z$ be a smooth real algebraic variety. Then $Z$ can be realized as the real points of a rational complex algebraic variety. This, actually, turns out to be false. Kollár calls it the shortest lived conjecture as it was stated in 1954 and disproved by Comessatti around 1914 (I don't remember the exact year). However, even if the statement is false, just the fact that it was made and no one realized for 50 years that it was false should show that these questions are by no means easy. (Comessatti's paper was in Italian and I have no idea how Kollár found it.)


Kollár showed more systematically the possible topology types of manifolds that can satisfy this statement. In particular, Kollár shows that any closed connected
3-manifold occurs as a possibly non-projective real variety birationally equivalent to $P^3$. In other words the way Nash's conjecture fails is on the verge of the difference between projective and proper again showing that these questions are not easy." (Sandor Kovacs)


As I have been even more concerned with, let me add a few comments. Comessatti's result was certainly "well known" at least to some Italian people and also to the real algebraic Russian guys of Arnol'd's and mostly Rokhlin's school. Moreover (before Kollar's work) it had been rediscovered (via modern tools)
for instance by R. Silhol. This allows the following 2D solution of the Nash conjecture:


" (1) Every non orientable compact closed surface admits a rational projective non singular model which can be explicitly given by (algebraically) blowing-up $\mathbb RP^2$ at some points;


(2) $S^2$ and $T^2$ are the only orientable surfaces that admit non singular projective rational models (Comessatti);


(3) Every surface $S$ admits a projective rational model possibly having one singular point (to get it, first smoothly blow-up $S$ at one point $p$ getting a
non orientable $S'$ containing a smooth exceptional curve $C$ over $p$. Take a projective non singular rational model $S'\_r$ of $S$.
Finally one can prove that $C$ can be approximated in $S'\_r$ by a non singular algebraic curve $C\_a$ and that we can perform a "singular" blow-down of $C\_a$ producing the required singular rational model of $S$.)"


Inspired by this 2D discussion, in the paper (with A. Marin) Dechirures de varietes de dimension trois et la conjecture de Nash de rationalite' en dimension trois Comment. Math. Helv. 67 (4) (1992), 514-545 (a PDF file is available in [http://www.dm.unipi.it/~benedett/research.html](http://www.dm.unipi.it/%7Ebenedett/research.html))


we got a formally similar 3D solution. More precisely we provide at first a complete classification of compact closed smooth 3-manifolds $M$ up to "flip/flop" along smooth centres (see below - these are the "Dechirures" - perhaps we were wrong to write the paper in French ... ).


Summarizing (and roughly) the results are:


* There is a complete invariant $I(M)$ for this equivalence relation. Depending only on $I(M)$, we explicitly produce a real projective non singular
rational 3-fold $Z$ such that $I(M)=I(Z)$.
* There is a smooth link $L$ in $M$ and a non singular real algebraic link $L\_a$ in $Z$ such that by smoothly (algebraically) blowing up $M (Z)$ along $L (L\_a)$
we get the same manifold $Z'$ and furthermore the disjoint real algebraic exceptional tori over $L\_a$ coincide with the exceptional tori over $L$ (thinking all within the smooth category, basically by definition this means that $Z$ and $M$ are equivalent up to flip/flop along smooth centres).
* Clearly $Z'$ is projective rational non singular and algebraically dominates $Z$. It also smoothly dominates M. Finally we can convert the smooth
blow-down onto $M$ to a singular algebraic blow-down producing a projective rational model $M\_r$ of $M$, possibly singular at a non singular
real algebraic copy of the link $L$ in $M\_r$.
* The invariant $I(M)$ is easy when $M$ is orientable; this is just the dimension of $H\_2(M;\mathbb Z/2)$. In this case $Z$ is obtained by algebraically blowing up
$S^3$ at some points. $I(M)$ is much more complicated if M is non orientable and involves, among other things, certain quadratic forms on
$Ker(i\_\*:H\_1(S;\mathbb Z/2) \to H\_1(M;\mathbb Z/2))$, $S$ being any characteristic surface of $M$.


A combination of our work with Kollar's one roughly gives:


* (3D "a la" Comessatti) In the projective framework, in general our *singular* rational models cannot be improved (singularity cannot be avoided, and in a sense we provided models with very mild singularities); in other words those blowing-down $Z'\to M\_r$ were intrinsically singular.
* (Non projective non singular rational models) Starting from our real projective rational non singular 3-fold $Z'$, as above, Kollar proves that one can realize a non singular blow-down $Z'\to M'\_r$, provided that we leave the projective framework.


Finally it is intriguing to note another important occurrence of the opposition projective singular vs non singular (related to the existence of intrinsically singular blow-down). Going back to the original Nash-Tognoli kind of problems, for a while it was conjectured and very "desidered" (for several reasons also related to the general question of characterizing say the compact polyhedron which admit possibly singular real algebraic models) that every M as above admits a "totally algebraic model" M\_a, i.e. such that $H\_\*(M\_a;\mathbb Z/2)$ is generated by the $\mathbb Z/2$-fundamental class of algebraic sub-varieties of $M\_a$. On the contrary we constructed counterexamples in:


(with M. Dedo') Counterexamples to representing homology classes by real algebraic subvarieties up to homeomorphism, Compositio Math. 53 (2) (1984), 143-151 (idem)


This contrasts with a result by Akbulut-King that M admits *singular* totally algebraic models.


