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FibonacciNumber.java

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package Maths;

/**

* Fibonacci: 0 1 1 2 3 5 8 13 21 ...

*/

public class FibonacciNumber {

public static void main(String[] args) {

assert isFibonacciNumber(1);

assert isFibonacciNumber(2);

assert isFibonacciNumber(21);

assert !isFibonacciNumber(9);

assert !isFibonacciNumber(10);

}

/**

* Check if a number is perfect square number

*

* @param number the number to be checked

* @return <tt>true</tt> if {@code number} is perfect square, otherwise <tt>false</tt>

*/

public static boolean isPerfectSquare(int number) {

int sqrt = (int) Math.sqrt(number);

return sqrt * sqrt == number;

}

/**

* Check if a number is fibonacci number

* This is true if and only if at least one of 5x^2+4 or 5x^2-4 is a perfect square

*

* @param number the number

* @return <tt>true</tt> if {@code number} is fibonacci number, otherwise <tt>false</tt>

* @link https://en.wikipedia.org/wiki/Fibonacci_number#Identification

*/

public static boolean isFibonacciNumber(int number) {

return isPerfectSquare(5 * number * number + 4) || isPerfectSquare(5 * number * number - 4);

}

}