First homology group of a double torus is $H_1(T^2\#T^2)=\mathbb Z^4,$ (where # stands for a connected sum) which – for me – intuitively means there are 4 different cycles up to homotopy, the black ones. But what about those two yellow? Are they some kind of combination of the four?
2026-03-29 12:05:26.1774785926
First homology group of a double torus (genus 2 surface) – intuition
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Here's some pictures that show how to get the big yellow loop from the two horizontal black loops and the smaller yellow loop from the two vertical black loops.
Big yellow:
Small yellow: