JavaAlgorithms/Maths/FibonacciNumber.java

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);
  }
}