Voor convexe lichamen geldt dat voor elke 2 punten A en B van dit lichaam ook elk ander punt tussen die punten tot het lichaam behoort. Concreet betekent dat dat er geen 'kuilen' in het lichaam zitten.
Concave lichamen zijn niet-convex, dus kun je ergens wel 2 punten A en B vinden zodat niet alle punten deel uitmaken van het lichaam.
'In three dimensions, any polyhedron whose faces are all regular pentagons may be stellated in the same way. The dodecahedron {5,3} (meaning that at each vertex we see 3 pentagons) gives rise to the stellated dodecahedron in this way. This polyhedron, sometimes called the small stellated dodecahedron for greater precision, has Schlafli symbol {5/2,5}, meaning that each vertex is surrounded by 5 pentagrams {5/2}, arranged pentagonally.
But we can also replace each face of the dodecahedron by the corresponding 'great face', namely the regular pentagon {5} whose vertices are the 5 vertices of one of these pentagrams. This results in the 'great dodecahedron', whose Schlafli symbol is {5,5/2} since at each vertex we have five pentagons {5} arranged 'pentagrammatically'. [These pentagons are the regular pentagons you can see in an icosahedron.]
We can now stellate the pentagons of a great dodecahedron to produce what can be called either the 'stellated great dodecahedron' or (more traditionally), the 'great stellated dodecahedron'. Both names are equally appropriate, since this can be produced either by 'greatening' the faces of the 'stellated dodecahedron' {5/2,3} or by 'stellating' the faces of the 'great dodecahedron' {5,5/2}.Its Schlafli symbol is {5/2,3}, since 3 pentagrams {5/2} meet at each vertex.
The dual of {5/2,3} is a polyhedron {3,5/2}, whose faces are 'great triangles' {3} lying in the same planes as the faces of the icosahedron {3,5}. It is therefore naturally called the
'great icosahedron'.
You will see from these systematic descriptions that it's better to omit the word 'small' from 'small stellated dodecahedron', since this is in fact the only 'stellated dodecahedron' per se.
The polyhedra {5/2,5} and {5/2,3} were discovered by Kepler, and their duals {5,5/2} and {3,5/2} by Poinsot. Cauchy proved that these four are the only finite regular star polyhedra.'
