TITLE: A topological consequence of Riemann-Roch in the almost complex case
QUESTION [18 upvotes]: This question originated from a conversation with Dmitry that took place here
Is there a complex structure on the 6-sphere?
The Hirzebruch-Riemann-Roch formula expresses the Euler characteristic of a holomorphic vector bundle on a compact complex manifold $M$ in terms of the Chern classes of the bundle and of the manifold. On the other hand, for complex manifolds the $\bar\partial$ operator is a differential (i.e. it squares to zero) and hence, the complex of sheaves of smooth $(p,q)$ forms for a fixed $p$ equipped with the $\bar\partial$ differential is a resolution of $\Omega^p$; this is a complex of soft sheaves and so, by taking global sections we can compute the cohomology of $\Omega^p$. Moreover, since the de Rham complex resolves the constant sheaf, the alternating sum of the Euler characteristics of $\Omega^p$'s is the Euler characteristic of $M$.
For an arbitrary pseudo-complex manifold the only part of the above that makes sense is the "right hand side" of the Riemann-Roch formula (i.e. the one involving Chern classes) and the (topological) Euler characteristic of the manifold itself. So it seems natural to ask whether the relation between the two that is true in the complex case remains true in the almost complex case. In other words, is it true that for a compact almost complex manifold $M$ of dimension $2n$ we have $$\chi(M)=\sum_{p=0}^n (-1)^p\sum_{i=0}^{n\choose{p}}\mathrm{ch}_{n-i}(\Omega^p)\frac{T_i}{i!}$$ where $\chi$ is the topological Euler characteristic, $\Omega^p$ is the $p$-th complex exterior power of the cotangent bundle (i.e., the complex dual of the tangent bundle), $\mathrm{ch}$ is the Chern character and $T_i$ is the $i$-th Todd polynomial of $M$?
In general, are there topological consequences of the existence of the Dolbeault resolution that would be difficult to prove (or, more ambitiously, would fail) for arbitrary pseudo-complex manifolds?

REPLY [3 votes]: I just wanted to point out how this question is related to (spin${}^c$) Dirac operators and their indicies since this was alluded to in the comments to the question.

Let $(M, g)$ be an $2n$-dimensional closed Riemannian manifold. Given a spin${}^c$ structure, one can form the complex spin${}^c$ bundles $\mathbb{S}^+_{\mathbb{C}}$ and $\mathbb{S}^-_{\mathbb{C}}$. For any hermitian vector bundle $E \to M$, there is a twisted spin${}^c$ Dirac operator $D^c_E : \Gamma(\mathbb{S}^+_{\mathbb{C}}\otimes E) \to \Gamma(\mathbb{S}^-_{\mathbb{C}}\otimes E)$ which has index
$$\operatorname{ind}(D^c_E) = \int_M\exp(c_1(L)/2)\operatorname{ch}(E)\hat{A}(TM)$$
where $L$ is the complex line bundle associated to the spin${}^c$ structure.
Suppose now that $M$ admits an almost complex structure $J$ and $g$ is hermitian. Then there is a canonical spin${}^c$ structure which has associated line bundle $L = \det_{\mathbb{C}}(TM)$, so $c_1(L) = c_1(M)$. Using the fact that $\exp(c_1(M)/2)\hat{A}(TM) = \operatorname{Td}(M)$, the index becomes
$$\operatorname{ind}(D^c_E) = \int_M \operatorname{ch}(E)\operatorname{Td}(M).$$
Moreover, $\mathbb{S}^+_{\mathbb{C}} \cong \bigwedge^{0,\text{even}}M$ and $\mathbb{S}^-_{\mathbb{C}} \cong \bigwedge^{0, \text{odd}}M$.
When $J$ is integrable and $E$ is holomorphic, $D^c_E = \sqrt{2}(\bar{\partial}_E + \bar{\partial}^*_E)$ and the above equality becomes the statement of the Hirzebruch-Riemann-Roch Theorem. In particular, if $E = \Omega^p$, then $D^c_p := D^c_{\Omega^p}$ has index $\operatorname{ind}(D^c_p) = \chi(M, \Omega^p)$.
In the general case, Hirzebruch defined $\chi^p(M) := \operatorname{ind}(D^c_p)$ and introduced the Hirzebruch $\chi_y$ genus $\chi_y(M) = \sum_{p=0}^n\chi^p(M)y^p$. With this notation, your question is whether or not the equality $\chi_{-1}(M) = \chi(M)$ holds. Using Chern roots $x_i$ of $TM$, one finds that
$$\chi_y(M) = \int_M\prod_{i=1}^n\frac{x_i(1+ye^{-x_i})}{1-e^{-x_i}}.$$
Setting $y = -1$, we conclude $\chi_{-1}(M) = \chi(M)$ exactly as in David E Speyer's answer.
See this note for more details on Hirzebruch's $\chi_y$ genus and its properties, as well as references for the claims made above.