nLab
Pontryagin class

Idea

The Pontryagin classes are characteristic classes on the classifying space $\mathcal{B}O(n)$ of the orthogonal group and, by pullback, on the base of any bundle with structural group the orthogonal group. The latter is where they were originally defined.

The analogs for the unitary group are the Chern classes.

Definition

The universal Pontryagin characteristic classes $P_k$ on the classifying space $B O(n)$ are, up to a sign, the pullbacks of the Chern classes $c_{2k}$ along the complexification inclusion

$$B O(n) \to B U(n) \,.$$

Properties

As generating universal characteristic classes

The torsion-free quotient of the cohomology ring $H^\bullet(B SO(2n+1), \mathbb{Z})$ is the polynomial ring on all Pontryagin classes $\{P_i\}_{i = 1}^n$. The torsion is generated by Bockstein images of $H^\bullet(BSO(2n+1);\mathbb{F}_2)$, which is generated by the Stiefel-Whitney classes.

The torsion-free quotient of the cohomology ring $H^\bullet(B SO(2n), \mathbb{Z})$ is the quotient of the polynomial ring on Pontryagin classes $P_i$ and the Euler class $\chi$ by the relation $\chi^2 = P_n$; again the torsion is generated by Bocksteins of monomials in the Stiefel–Whitney classes.

Further relation to Chern classes

Under the other canonical map

$$j \;\colon\; B U(n) \to BO(2n)$$

one has

$$j^\ast(P_k) = \sum_{a + b = 2 k} (-1)^{a+k} c_a c_b$$

and

$$j^\ast(\chi) = c_n \,.$$

Splitting principle and Chern roots

Under the inclusion

$$i \;\colon\; U(1)^n \hookrightarrow U(n) \to O(2n)$$

of the maximal torus one has that

$$(B i)^\ast(P_k) = \sigma_k(x_1, \cdots, x_n)^2$$

and

$$(B i)^\ast(\chi) = \sigma_n(x_1, \cdots, x_n)$$

where the $x_i \in H^\bullet(B U(1)^n, \mathbb{Z})$ are the “Chern roots”.

See at Chern class - Properties – Splitting principle and Chern roots and at splitting principle - Examples - Real vector bundles for more.

Trivializations and structures

The twisted differential c-structures corresponding to Pontryagin class include

• twisted differential string structure for the first fractional Pontryagin class $\frac{p_1}{2} : B Spin \to B^3 U(1)$;

• twisted differential fivebrane structure for the second fractional Pontryagin class $\frac{1}{6}p_2 : B String \to B^7 U(1)$.

Related concepts

• Pontryagin class

• p1-structure, string structure, fivebrane structure

• Wu class

• Chern class

• Stiefel-Whitney class

• Chern character

• orthogonal calculus

References

Classical textbook references are

• John Milnor, Jim Stasheff, Characteristic classes, Princeton Univ. Press

• Werner Greub, Stephen Halperin, Ray Vanstone, Connections, Curvature, and Cohomology Academic Press (1973)

See also

• Paul Bressler, The first Pontryagin class, math.AT/0509563

• Ivan Panin, Charles Walter, Quaternionic Grassmannians and Pontryagin classes in algebraic geometry, arxiv/1011.0649

A brief introduction is in chapter 23, section 7

• Peter May, A concise course in algebraic topology (pdf)