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Encyclopedic overview
In mathematics, a polygonal number is a number that counts dots arranged in the shape of a regular polygon. These are one type of 2-dimensional figurate numbers.
Polygonal numbers were first studied during the 6th century BC by the Ancient Greeks, who investigated and discussed properties of oblong, triangular, and square numbers.
Excerpted from Wikipedia’s “polygonal number” article, available under the CC BY-SA 4.0 licence.