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triangular number
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Oblong numbers formed by doubling triangular numbers Diagram showing four types of polygonal numbers with colored dots arranged in geometric patterns: blue triangles for triangular numbers, green squares for square numbers, red pentagons for pentagonal numbers, and purple hexagons for hexagonal numbers, each illustrating how numbers like 1, 3, 6, 10, 4, 9, 16, 5, 12, 22, 6, 15, and 28 can be visually represented. Square numbers shown formed from consecutive triangular numbers Figure 1: Polygonal arrays.
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Oblong numbers formed by doubling triangular numbers
Oblong numbers formed by doubling triangular numbers Figure 2: Oblong numbers formed by doubling triangular numbers.

Triangular numbers are numbers that can be represented by dots arranged in triangles. These numbers are formed by the successive sums of the first n natural numbers, following the formula ( \frac{n(n + 1)}{2} ), where n represents a positive integer. The sequence begins with 1, 3, 6, 10, 15, and so on.12

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