A polygonal diagonal is a line segment connecting two nonadjacent polygon vertices of a polygon.
The number of ways a fixed convex -gon can be divided into triangles
by nonintersecting diagonals is (with diagonals), where is a Catalan number. This
is Euler's polygon division problem.
Counting the number of regions determined by drawing the diagonals of a regular -gon is a more difficult problem, as is
determining the number of -tuples of concurrent diagonals
(Kok 1972).

The number of regions which the diagonals of a convex
polygon divide its center if no three are concurrent in its interior is

(1)

(2)

The first few values are 0, 0, 1, 4, 11, 25, 50, 91, 154, 246, ... (OEIS A006522).

Kok, J. Item 2 in Beeler, M.; Gosper, R. W.; and Schroeppel, R. HAKMEM. Cambridge, MA: MIT Artificial Intelligence Laboratory, Memo AIM-239,
p. 3, Feb. 1972. http://www.inwap.com/pdp10/hbaker/hakmem/geometry.html#item2.Sloane,
N. J. A. Sequence A006522/M3413
in "The On-Line Encyclopedia of Integer Sequences."