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  2. Triangular number - Wikipedia

    en.wikipedia.org/wiki/Triangular_number

    For example, the sixth heptagonal number (81) minus the sixth hexagonal number (66) equals the fifth triangular number, 15. Every other triangular number is a hexagonal number. Knowing the triangular numbers, one can reckon any centered polygonal number; the n th centered k-gonal number is obtained by the formula = + where T is a triangular ...

  3. Polygonal number - Wikipedia

    en.wikipedia.org/wiki/Polygonal_number

    The triangular number sequence is the representation of the numbers in the form of equilateral triangle arranged in a series or sequence. These numbers are in a sequence of 1, 3, 6, 10, 15, 21, 28, 36, 45, and so on.

  4. 15 (number) - Wikipedia

    en.wikipedia.org/wiki/15_(number)

    the first number to be polygonal in 3 ways: it is the 5th triangular number, [5] a hexagonal number, [6] and pentadecagonal number. [7] a centered tetrahedral number. the number of partitions of 7. the smallest number that can be factorized using Shor's quantum algorithm. the magic constant of the unique order-3 normal magic square.

  5. List of types of numbers - Wikipedia

    en.wikipedia.org/wiki/List_of_types_of_numbers

    Such a number is algebraic and can be expressed as the sum of a rational number and the square root of a rational number. Constructible number: A number representing a length that can be constructed using a compass and straightedge. Constructible numbers form a subfield of the field of algebraic numbers, and include the quadratic surds.

  6. 1 + 2 + 3 + 4 + ⋯ - Wikipedia

    en.wikipedia.org/wiki/1_%2B_2_%2B_3_%2B_4_%2B_%E...

    The first six triangular numbers. The partial sums of the series 1 + 2 + 3 + 4 + 5 + 6 + ⋯ are 1, 3, 6, 10, 15, etc.The nth partial sum is given by a simple formula

  7. Figurate number - Wikipedia

    en.wikipedia.org/wiki/Figurate_number

    Speusippus is the earliest source to expose the view that ten, as the fourth triangular number, ... the figure illustrates 1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 = 64 = 8 2.

  8. Tetrahedral number - Wikipedia

    en.wikipedia.org/wiki/Tetrahedral_number

    Tetrahedral numbers can be modelled by stacking spheres. For example, the fifth tetrahedral number (Te 5 = 35) can be modelled with 35 billiard balls and the standard triangular billiards ball frame that holds 15 balls in place. Then 10 more balls are stacked on top of those, then another 6, then another three and one ball at the top completes ...

  9. 120 (number) - Wikipedia

    en.wikipedia.org/wiki/120_(number)

    the fifteenth triangular number, [2] as well as the sum of the first eight triangular numbers, making it also a tetrahedral number. 120 is the smallest number to appear six times in Pascal's triangle (as all triangular and tetragonal numbers appear in it). Because 15 is also triangular, 120 is a doubly triangular number. 120 is divisible by the ...