triangular number

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Oblong numbers formed by doubling triangular numbers

polygonal numbers

Square numbers shown formed from consecutive triangular numbers

Figure 1: Polygonal arrays.

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triangular number

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definition and properties

In number game: Polygonal and other figurate numbers
” Thus, the triangular numbers, 1, 3, 6, 10, 15, 21, etc., were visualized as points or dots arranged in the shape of a triangle.
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