Algebraic Topology
Lesson 1 of 3 11 min +120 XP

Homotopy & the Fundamental Group

Shape up to continuous deformation.

What you'll learn

  • Describe a homotopy
  • Define the fundamental group informally
  • Distinguish spaces by π₁
The lasso around the pole

Loop a rope on an open field and you can always reel it back to your hand. Loop it around a lamppost and it snags — no shrinking gets it free. The fundamental group counts exactly these kinds of loops a space traps.

Deforming without tearing

Two loops are homotopic if one can be continuously deformed into the other. The fundamental group π₁ collects loops up to homotopy, capturing how a space is 'connected'.

Holes matter

A disk has trivial π₁ — every loop shrinks to a point. A circle does not: a loop around it can't be undone, so π₁(S¹) ≅ ℤ.

Knowledge Check

+30 XP / correct

1. The fundamental group of the circle S¹ is isomorphic to…

2. Two loops are homotopic if…