Decagonal number

In mathematics, a decagonal number is a figurate number that extends the concept of triangular and square numbers to the decagon (a ten-sided polygon). However, unlike the triangular and square numbers, the patterns involved in the construction of decagonal numbers are not rotationally symmetrical. Specifically, the n-th decagonal numbers counts the dots in a pattern of n nested decagons, all sharing a common corner, where the ith decagon in the pattern has sides made of i dots spaced one unit apart from each other. The n-th decagonal number is given by the following formula

$d_{n}=4n^{2}-3n$.

The first few decagonal numbers are:

0, 1, 10, 27, 52, 85, 126, 175, 232, 297, 370, 451, 540, 637, 742, 855, 976, 1105, 1242, 1387, 1540, 1701, 1870, 2047, 2232, 2425, 2626, 2835, 3052, 3277, 3510, 3751, 4000, 4257, 4522, 4795, 5076, 5365, 5662, 5967, 6280, 6601, 6930, 7267, 7612, 7965, 8326 (sequence A001107 in the OEIS).

The nth decagonal number can also be calculated by adding the square of n to thrice the (n−1)th pronic number or, to put it algebraically, as

$D_{n}=n^{2}+3\left(n^{2}-n\right)$.

Properties

• Decagonal numbers consistently alternate parity.
• $D_{n}$ is the sum of the first $n$ natural numbers congruent to 1 mod 8.
• $D_{n}$ is number of divisors of $48^{n-1}$.
• The only decagonal numbers that are square numbers are 0 and 1.
• The decagonal numbers follow the following recurrence relations:

$D_{n}=D_{n-1}+8n-7,D_{0}=0$

$D_{n}=2D_{n-1}-D_{n-2}+8,D_{0}=0,D_{1}=1$

$D_{n}=3D_{n-1}-3D_{n-2}+D_{n-3},D_{0}=0,D_{1}=1,D_{2}=10$