# Octagonal number

• Difficulty Level : Basic
• Last Updated : 16 Jul, 2021

You are given a number n, the task is to find nth octagonal number. Also, find the Octagonal series till n.
An octagonal number is the figure number that represent octagonal. Octagonal numbers can be formed by placing triangular numbers on the four sides of a square. Octagonal number is calculated by using the formula (3n2 – 2n)
Examples :

```Input : 5
Output : 65

Input : 10
Output : 280

Input : 15
Output : 645```

## C++

 `// C++ program to find``// nth octagonal number``#include ``using` `namespace` `std;` `// Function to calculate``//octagonal number``int` `octagonal(``int` `n)``{``    ``// Formula for finding``    ``// nth octagonal number``    ``return` `3 * n * n - 2 * n;``}` `// Driver function``int` `main()``{``    ``int` `n = 10;``    ``cout << n << ``"th octagonal number :"``         ``<< octagonal(n);``    ``return` `0;``}`

## Java

 `// Java program to find``// nth octagonal number``import` `java.util.*;``import` `java.lang.*;` `public` `class` `GfG {` `    ``// Function to calculate``    ``//octagonal number``    ``public` `static` `int` `octagonal(``int` `n)``    ``{``        ``// Formula for finding``        ``// nth octagonal number``        ``return` `3` `* n * n - ``2` `* n;``    ``}` `    ``// Driver function``    ``public` `static` `void` `main(String argc[])``    ``{``        ``int` `n = ``10``;``        ``System.out.println(n + ``"th octagonal"` `+``                     ``" number :"` `+ octagonal(n));``    ``}``}` `/* This code is contributed by Sagar Shukla */`

## Python

 `# Python program to find``# nth octagonal number``def` `octagonal(n):``    ``return` `3` `*` `n ``*` `n ``-` `2` `*` `n` `# Driver code``n ``=` `10``print``(n, ``"th octagonal number :"``,``       ``octagonal(n))`

## C#

 `// C# program to find nth octagonal number``using` `System;` `public` `class` `GfG {` `    ``// Function to calculate``    ``//octagonal number``    ``public` `static` `int` `octagonal(``int` `n)``    ``{``        ` `        ``// Formula for finding``        ``// nth octagonal number``        ``return` `3 * n * n - 2 * n;``    ``}` `    ``// Driver function``    ``public` `static` `void` `Main()``    ``{``        ``int` `n = 10;``        ` `        ``Console.WriteLine(n + ``"th octagonal"``              ``+ ``" number :"` `+ octagonal(n));``    ``}``}` `/* This code is contributed by Vt_m */`

## PHP

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## Javascript

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Output :

`10th octagonal number : 280`

Time Complexity: O(1)
Auxiliary Space: O(1)

#### Given number n, find the octagonal series till n.

We can also find the octagonal series. Octagonal series contains the points on octagonal.

`Octagonal series 1, 8, 21, 40, 65, 96, 133, 176, 225, 280, . . .`

## C++

 `// C++ program to display the``// octagonal series``#include ``using` `namespace` `std;` `// Function to display``// octagonal series``void` `octagonalSeries(``int` `n)``{``    ``// Formula for finding``    ``//nth octagonal number``    ``for` `(``int` `i = 1; i <= n; i++)` `        ``// Formula for computing``        ``// octagonal number``        ``cout << (3 * i * i - 2 * i);   ``}` `// Driver function``int` `main()``{``    ``int` `n = 10;``    ``octagonalSeries(n);``    ``return` `0;``}`

## Java

 `// Java program to find``// nth octagonal number``import` `java.util.*;``import` `java.lang.*;`` ` `public` `class` `GfG {`` ` `    ``// Function to display octagonal series``    ``public` `static` `void` `octagonalSeries(``int` `n)``    ``{``        ``// Formula for finding``        ``//nth octagonal number``        ``for` `(``int` `i = ``1``; i <= n; i++)`` ` `            ``// Formula for computing``            ``// octagonal number``            ``System.out.print(``3` `* i * i - ``2` `* i);``    ``}`` ` `    ``// Driver function``    ``public` `static` `void` `main(String argc[])``    ``{``        ``int` `n = ``10``;``        ``octagonalSeries(n);``    ``}`` ` `    ``/* This code is contributed by Sagar Shukla */``}`

## Python

 `# Python program to find``# nth octagonal number``def` `octagonalSeries(n):``    ``for` `i ``in` `range``(``1``, n ``+` `1``):``        ``print``(``3` `*` `i ``*` `i ``-` `2` `*` `i,``                   ``end ``=` `", "``)` `# Driver code``n ``=` `10``octagonalSeries(n)`

## C#

 `// C# program to find``// nth octagonal number``using` `System;` `public` `class` `GfG {` `    ``// Function to display octagonal series``    ``public` `static` `void` `octagonalSeries(``int` `n)``    ``{``        ` `        ``// Formula for finding``        ``//nth octagonal number``        ``for` `(``int` `i = 1; i <= n; i++)` `            ``// Formula for computing``            ``// octagonal number``            ``Console.Write(3 * i * i - 2 * i + ``", "``);``    ``}` `    ``// Driver function``    ``public` `static` `void` `Main()``    ``{``        ``int` `n = 10;``        ` `        ``octagonalSeries(n);``    ``}``}` `/* This code is contributed by Vt_m */`

## PHP

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## Javascript

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Output :

`1, 8, 21, 40, 65, 96, 133, 176, 225, 280`

Time Complexity: O(n)
Auxiliary Space: O(1)

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