Courses from 1000+ universities
AI got cheap enough that Duolingo’s most expensive plan may not survive it. I read the earnings call transcript and opened the app to see what is actually changing for learners.
600 Free Google Certifications
Artificial Intelligence
IT & Networking
Software Engineering
Supporting Victims of Domestic Violence
Know Thyself - The Value and Limits of Self-Knowledge: The Examined Life
Understanding Dementia
Organize and share your learning with Class Central Lists.
View our Lists Showcase
An introduction to spherical geometry using great circles, antipodal points, spherical triangles, and circles on the sphere, illustrated with physical models.
A careful mathematical study of decimal notation, including how decimal expansions represent fractions and how to convert them systematically.
Learn how homotopic paths and based loops combine to define fundamental groups, with examples from the disk and circle.
An introduction to knot theory using Reidemeister moves and basic invariants including crossing number, linking number, and the Alexander–Conway polynomial.
Explore hyperbolic structures on a two-holed torus and a 3-crosscaps surface through hexagon and octagon models and plane tessellations.
Uses the degree of a map from the circle to prove results about disk retractions, fixed points, and antipodes.
Explore non-orientable surfaces through the Möbius band, its single edge, chirality, crosscaps, and deformation into related models.
Explore the five Platonic solids and prove Euler’s formula for polyhedra using triangulation and flow lines on a sphere.
Introduces cylinders, tori, and n-holed tori, defining surface genus and examining plane coverings and their associated translation groups.
Explore how Babylonian, Egyptian, Chinese, Greek, and Roman cultures developed number systems, from primitive counting to the Hindu-Arabic system.
Explores and proves the Triple quad formula in universal hyperbolic geometry, relating quadrances through a challenging polynomial identity.
Explore the hyperbolic version of Pythagoras’ theorem through quadrance, planar coordinates, examples, exercises, and a proof.
Explore quadrance and spread as algebraic metrics, then compare rational trigonometry in Euclidean affine geometry with universal hyperbolic geometry.
Connects the J function and 2×2 matrix Lie brackets to the Jacobi identity, yielding a projective proof of a triangle’s orthocenter.
Explores parallels in Universal Hyperbolic Geometry through double-triangle constructions and associated perspective centers.
Get personalized course recommendations, track subjects and courses with reminders, and more.