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

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, 141, ..

Examples :

```Input : 8
Output : Yes

Input : 34
Output : Yes

Input : 41
Output : No
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

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).

 `// Java program to check if x is a perfect square ` ` `  `class` `GFG { ` `    ``// A utility method that returns true if x is perfect square ` `    ``static` `boolean` `isPerfectSquare(``int` `x) ` `    ``{ ` `        ``int` `s = (``int``)Math.sqrt(x); ` `        ``return` `(s * s == x); ` `    ``} ` ` `  `    ``// Returns true if n is a Fibonacci Number, else false ` `    ``static` `boolean` `isFibonacci(``int` `n) ` `    ``{ ` `        ``// n is Fibonacci if one of 5*n*n + 4 or 5*n*n - 4 or both ` `        ``// is a perfect square ` `        ``return` `isPerfectSquare(``5` `* n * n + ``4``) || isPerfectSquare(``5` `* n * n - ``4``); ` `    ``} ` ` `  `    ``// Driver method ` `    ``public` `static` `void` `main(String[] args) ` `    ``{ ` `        ``for` `(``int` `i = ``1``; i <= ``10``; i++) ` `            ``System.out.println(isFibonacci(i) ? i + ``" is a Fibonacci Number"` `                                              ``: i + ``" is a not Fibonacci Number"``); ` `    ``} ` `} ` `// This code is contributed by Nikita Tiwari `

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!

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