: Mathematics: centered triangular number

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centered triangular number

A centered (or centred) triangular number is a centered figurate number that represents an equilateral triangle with a dot in the center and all its other dots surrounding the center in successive equilateral triangular layers.
This is also the number of points of a hexagonal lattice with nearest-neighbor coupling whose distance from a given point is less than or equal to ${\\displaystyle n}$ .
(Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/wiki/Centered_triangular_number)

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