The exchange axiom for pregeometries is sometimes called the Steinitz exchange axiom (after its specialization to vector spaces, where it is sometimes known as the Steinitz exchange lemma.)

]]>Two days ago, I created

One reason was (I keep this notification short) something like

I am working, for pasting schemes, on *plane digraphs* -> working with embeddings becomes important -> a central theorem about plane graphs is of course Whitney’s theorem about unique embeddability of 3-connected planar graphs -> arguably the most well-known, but heretofore not nLab-documented theorem about planar 3-connected graphs is a theorem of Steinitz’s.