# Python Program for How to check if a given number is Fibonacci number?

• Difficulty Level : Easy
• Last Updated : 07 Jul, 2021

Given a number \’n\’, how to check if n is a Fibonacci number. First few Fibonacci numbers are 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, ..
Examples :

```Input : 8
Output : Yes

Input : 34
Output : Yes

Input : 41
Output : No```

Following is an interesting property about Fibonacci numbers that can also be used to check if a given number is Fibonacci or not.
A number is Fibonacci if and only if one or both of (5*n2 + 4) or (5*n2 – 4) is a perfect square (Source: Wiki).

## Python

 `# python program to check if x is a perfect square``import` `math` `# A utility function that returns true if x is perfect square``def` `isPerfectSquare(x):``    ``s ``=` `int``(math.sqrt(x))``    ``return` `s``*``s ``=``=` `x` `# Returns true if n is a Fibinacci Number, else false``def` `isFibonacci(n):` `    ``# n is Fibinacci if one of 5*n*n + 4 or 5*n*n - 4 or both``    ``# is a perferct square``    ``return` `isPerfectSquare(``5``*``n``*``n ``+` `4``) ``or` `isPerfectSquare(``5``*``n``*``n ``-` `4``)``   ` `# A utility function to test above functions``for` `i ``in` `range``(``1``,``11``):``     ``if` `(isFibonacci(i) ``=``=` `True``):``         ``print` `i,``"is a Fibonacci Number"``     ``else``:``         ``print` `i,``"is a not Fibonacci Number "`

Output:

```1 is a Fibonacci Number
2 is a Fibonacci Number
3 is a Fibonacci Number
4 is a not Fibonacci Number
5 is a Fibonacci Number
6 is a not Fibonacci Number
7 is a not Fibonacci Number
8 is a Fibonacci Number
9 is a not Fibonacci Number
10 is a not Fibonacci Number ```

Please refer complete article on How to check if a given number is Fibonacci number? for more details!

My Personal Notes arrow_drop_up