Nonagonal number

A nonagonal number is a figurate number that represents a nonagon, which is a polygon with nine sides. Nonagonal numbers can be calculated using the formula: \[ N_n = \frac{n(7n - 5)}{2} \] where \( N_n \) is the \( n \)-th nonagonal number and \( n \) is a positive integer representing the position in the sequence of nonagonal numbers.