# Resources

-   Milnor and Stasheff, _Characteristic classes_.  
    
-   Hatcher, [_Vector bundles and K-theory_](https://www.math.cornell.edu/~hatcher/VBKT/VB.pdf)
-   Hutchings, [_Introduction to higher homotopy groups and obstruction theory_](https://akramalishahi.github.io/Spring2018/homotopy.pdf)
-   Hutchings, [_Cup products and intersections_](https://akramalishahi.github.io/Spring2018/cup.pdf)

## Useful Facts

- Any morphism between principal $G\dash$bundles is an isomorphism.
- A principal $G\dash$bundle is trivial iff it admits a section.