Euler's Formula

For any polyhedron that doesn't intersect itself, the
Number of Faces
plus the Number of Vertices (corner points)
minus the Number of Edges always equals 2

This can be written: F + V - E = 2

Try it on the cube:
A cube has 6 Faces, 8 Vertices, and 12 Edges,
so:
6 + 8 - 12 = 2

 Angles:

 Acute Angle== an angle that is less than 90°
Right Angle== an angle that is 90° exactly
Obtuse Angle== an angle that is greater than 90° but less than 180°
Straight Angle== an angle that is 180° exactly
Reflex Angle== an angle that is greater than 180°

 Jordan:

simple closed curve: a closed curve that does not intersect itself.

 Power Set:   

 The power set is the set of all subsets of a given set.  

And the empty set {} is also a subset of {a,b,c}

In fact, if you list all the subsets of S={a,b,c} you will have the Power Set of {a,b,c}:
P(S) = { {}, {a}, {b}, {c}, {a, b}, {a, c}, {b, c}, {a, b, c} }