Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

In Hatcher's Algebraic Topology, on page 246 (here, in the book), about two-thirds down the page he states that showing the commutativity of the two squares shown in the diagram, not involving the boundary or coboundary maps, is a 'triviality'.

enter image description here

I can sort of see how it works, but I simply can't work out the details satisfactorily, and all the while the word 'triviality' has been corroding my peace-of-mind like acid rain on a gargoyle. I don't suppose somebody could help me out?

share|cite|improve this question

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Browse other questions tagged or ask your own question.