12 Answers votes newest views recent 11“Philosophical” meaning of the Yoneda Lemma 3What’s a groupoid? What’s a good example of a groupoid? 6Resources for graphical languages / Penrose notation / Feynman diagrams / birdtracks? 2Category theory sans (much) motivation? 1What are examples of good toy models in mathematics? 0creating high quality figures of surfaces 8Is there a standard notation for binary relations in category theory? 2Understanding Penrose diagrammatical notation 3conditions for natural transformations to exist? 0When are all split monomorphisms complemented? 3When do PROP-morphisms induce adjunctions? 2Can the objects of every concrete category themselves be realized as small categories?