Dicuil (9th century) on triangular and square numbers

What is it about?

Triangular numbers are the number of dots that form an equilateral triangle. They are calculated as the sum of the natural numbers, or by more complex multiplication formulae. These formulae were known to the Greeks but were not well known in the West till the 16th century. Dicuil was a 9th century Irish monk who wrote an astronomical treatise and included a section on triangular and square numbers which described the complex formulae. He thus fills a gap in medieval knowledge.

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Why is it important?

Adds to medieval history of mathematics.