0
$\begingroup$

I was just reading about polyhedrons and wondered, even though it's an impossible shape, what a 3-hedron is called? I'm really asking what the Greek for '3'. Does it just converge to a triangle?

  • 0
    ["tri"](http://www.merriam-webster.com/dictionary/tri) seems like the right prefix, coming from Latin and Greek.2012-01-08
  • 0
    That was my first thought, though thought 'tri' was only Latin.2012-01-08
  • 2
    A relevant reference: http://en.wikipedia.org/wiki/Number_prefix2012-01-08
  • 0
    how about three-hedron? lol2012-01-08
  • 0
    I call him Harvey2012-01-08
  • 0
    Do you mean a [tetrahedron](http://en.wikipedia.org/wiki/Tetrahedron)?2012-01-08
  • 2
    @J.D.: No, tetrahedrons have 4 faces, not 3. The question is at face value asking about an object that doesn't exist, but is really asking about a prefix meaning $3$.2012-01-08
  • 0
    @JonasMeyer, it does exist, for appropriate values of «exist». An orthant deserves to be called a trihedron!2012-01-08
  • 0
    In the right ambient space, a trihedron makes sense and can exist. It's just as in spherical geometry (on the two-sphere), where you can start a figure with an angle at the north pole by drawing two longitude lines southward that will meet again at the south pole at the same angle. So in $S^3$, you can draw a trihedral angle at one pole, and continue the three “planes” to the opposite pole, where they will meet again.2012-01-08

1 Answers 1

4

Tri- is the greek prefix.

We do use the word trihedron, but to refer to something slightly different: the Frenet-Serret trihedron.