Classifying space for O(n) explained

In mathematics, the classifying space for the orthogonal group O(n) may be constructed as the Grassmannian of n-planes in an infinite-dimensional real space

Rinfty

.

Cohomology ring

The cohomology ring of

\operatorname{BO}(n)

with coefficients in the field

Z2

of two elements is generated by the Stiefel–Whitney classes:[1] [2]
*(\operatorname{BO}(n);Z
H
2) =Z

2[w1,\ldots,wn].

Infinite classifying space

The canonical inclusions

\operatorname{O}(n)\hookrightarrow\operatorname{O}(n+1)

induce canonical inclusions

\operatorname{BO}(n)\hookrightarrow\operatorname{BO}(n+1)

on their respective classifying spaces. Their respective colimits are denoted as:

\operatorname{O} :=\limn → infty\operatorname{O}(n);

\operatorname{BO} :=\limn → infty\operatorname{BO}(n).

\operatorname{BO}

is indeed the classifying space of

\operatorname{O}

.

See also

Literature

External links

References

  1. Milnor & Stasheff, Theorem 7.1 on page 83
  2. Hatcher 02, Theorem 4D.4.