Lab V · 射影面

The plane that
has no inside.

Boy’s surface is the real projective plane immersed smoothly in three-space — no edges, no creases, no points of singularity. Just one surface that, like the Möbius strip, refuses to choose a side.

Werner Boy proved its existence in 1901, against Hilbert’s expectation that no such immersion could exist. This is the minimal Bryant–Kusner version — the one that bends as little as possible.