Fundamental Polygon

In mathematics, each closed surface in the sense of geometric topology can be constructed from an even-sided oriented polygon, called a fundamental polygon, by pairwise identification of its edges. This construction can be represented as a string of length 2n of n distinct symbols where each symbol appears twice with exponent either +1 or -1. The exponent -1 signifies that the corresponding edge has the orientation opposing the one of the fundamental polygon.

Examples

Standard fundamental polygons

An orientable closed surface of genus n has the following standard fundamental polygon:
A_1 B_1 A_1^{-1} B_1^{-1}A_2 B_2 A_2^{-1} B_2^{-1}\cdots A_n B_n A_n^{-1} B_n^{-1}
A non-orientable closed surface of (non-orientable) genus n has the following standard fundamental polygon:
A_1 A_1 A_2 A_2 \cdots A_n A_n

 

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