The Hirzebruch–Riemann–Roch theorem applies to any holomorphic vector bundleE on a compact complex manifoldX, to calculate the holomorphic Euler characteristic of E in sheaf cohomology, namely the alternating sum of the dimensions as complex vector spaces. Hirzebruch's theorem states that χ is computable in terms of the Chern classes Cj of E, and the Todd polynomials Tj in the Chern classes of the holomorphic tangent bundle of X. These all lie in the cohomology ring of X; by use of the fundamental class we can obtain numbers from classes in The Hirzebruch formula asserts that taken over all relevant j, using the Chern character ch in cohomology. In other words, the cross products are formed in the cohomology ring of all the 'matching' degrees that add up to 2n, where to 'massage' the Cj a formal manipulation is done, setting and the total Chern class Formulated differently the theorem gives the equality where td is the Todd class of the tangent bundle of X. Significant special cases are when E is a complex line bundle, and when X is an algebraic surface. Weil's Riemann–Roch theorem for vector bundles on curves, and the Riemann–Roch theorem for algebraic surfaces, are included in its scope. The formula also expresses in a precise way the vague notion that the Todd classes are in some sense reciprocals of characteristic classes.
For curves, the Hirzebruch–Riemann–Roch theorem is essentially the classical Riemann–Roch theorem. To see this, recall that for each divisorD on a curve there is an invertible sheaf O such that the linear system of D is more or less the space of sections of O. For curves the Todd class is and the Chern character of a sheaf O is just 1+c1, so the Hirzebruch–Riemann–Roch theorem states that But h0 is just l, the dimension of the linear system of D, and by Serre dualityh1 = h0 = l where K is the canonical divisor. Moreover, c1 integrated over X is the degree of D, and c1 integrated over X is the Euler class 2 − 2g of the curveX, where g is the genus. So we get the classical Riemann Roch theorem For vector bundles V, the Chern character is rank + c1, so we get Weil's Riemann Roch theorem for vector bundles over curves:
Riemann Roch theorem for surfaces
For surfaces, the Hirzebruch–Riemann–Roch theorem is essentially the Riemann–Roch theorem for surfaces combined with the Noether formula. If we want, we can use Serre duality to express h2 as h0, but unlike the case of curves there is in general no easy way to write the h1 term in a form not involving sheaf cohomology.
Asymptotic Riemann-Roch
Let D be an ample Cartier divisor on an irreducible projective varietyX of dimension n. Then More generally, if is any coherent sheaf on X then