Cartographic Projection
every map is wrong, and by exactly how much
Concept

Heuristic — where it appears

A rule that finds a good answer without a guarantee that it is the best one. Its value is measurable where the best is known and unmeasurable above that size, and the two cases look identical from inside a sample of the search space.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Two unfoldings of the same 80-face solid. Both are edge unfoldings of the same solid along different spanning trees of its face graph, so both preserve every distance on the surface exactly. The left one is a net. The right one is not: 3 pairs of its faces occupy the same ground, so it cannot be cut out of paper and folded up. Nothing in the unfolding procedure prevents this, and past the regular solids most trees produce it.

A net can land on top of itself

Every one of the cube's 384 unfoldings is a net, and every one of the icosahedron's five million is too. Past the regular solids that stops being true: at 180 faces, 99 of every 100 randomly chosen unfoldings have faces sitting on top of each other, so choosing a net stops being a choice and becomes a search — except that the net anybody would actually draw works every time.

families · Polyhedral Every one of the cube's 384 nets, scored. All 384 spanning trees of the cube's face graph, unfolded and scored on the total separation their cuts leave: how far apart, in edge lengths, the two copies of each cut edge end up on the page. Every one of them is a valid net — no Platonic unfolding overlaps — and they range from 11.30 to 17.97, a factor of 1.59. The heuristic of unfolding outwards from a chosen face lands on the first of them, exactly.

What the net heuristic cannot find

Rung seven chose a net by unfolding outwards from a face and showed the choice beats guessing, and recorded that its own limit was unknown. Enumerated in full, the heuristic turns out to be exactly optimal on every solid small enough to check — rank one of 384 — and above that ceiling no sample can tell whether it still is, because eight hundred random nets never reach it.

families · Polyhedral

Named alongside it

The objects these essays reach for when they reach for this one.

CombinatoricsPlatonic solidPolyhedral projectionSearchSpanning treeCertificateCut locusEnumerationFaceGoldberg polyhedronIcosahedronInterruption

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