public static void IsSeriesFibonacci(int[] arr)
{
    var isFibonacci = true;
    var arrCount = arr.Length;

    if (arrCount <= 2)
    {
        Console.WriteLine($"Is the series [{string.Join(',', arr)}] Fibonacci? Array too small");
        return;
    }

    for (var i = 2; i < arrCount; i++)
    {
        if (arr[i] == arr[i - 1] + arr[i - 2]) continue;
        isFibonacci = false;
        break;
    }

    Console.WriteLine($"Is the series [{string.Join(',', arr)}] Fibonacci? {isFibonacci}");
}

public static void IsNumberFibonacci(int number)
{
    var fib1 = 0;
    var fib2 = 1;
    var fib3 = 0;

    while (fib3 < number)
    {
        fib3 = fib1 + fib2;
        fib1 = fib2;
        fib2 = fib3;
    }

    if (fib3 == number)
    {
        Console.WriteLine($"{number} is a Fibonacci number.");
    }
    else
    {
        Console.WriteLine($"{number} is not a Fibonacci number.");
    }
}

public static void FindNthFibonacciNumber(int number)
{
    var fib1 = 0;
    var fib2 = 1;
    var fib3 = 0;

    switch (number)
    {
        case 0 : Console.WriteLine($"The Fibonacci starts with the number {fib1}");
            return;
        case 1 : Console.WriteLine($"The first Fibonacci number is {fib1}");
            return;
        case 2 : Console.WriteLine($"The second Fibonacci number is {fib2}");
            return;
    }
    
    for (var i = 2; i <= number-1; i++)
    {
        fib3 = fib1 + fib2;
        fib1 = fib2;
        fib2 = fib3;
    }

    Console.WriteLine($"The {number}th Fibonacci number is {fib3}");
}

public static void FindFibonacciNumbersInRange(int start, int end)
{
    var fibonacciNumbers = new List<int>();
    var fib1 = 0;
    var fib2 = 1;
    var fib3 = 0;

    fibonacciNumbers.Add(fib1);
    fibonacciNumbers.Add(fib2);

    while (fib3 <= end)
    {
        fib3 = fib1 + fib2;
        fib1 = fib2;
        fib2 = fib3;

        fibonacciNumbers.Add(fib3);
    }

    var results = fibonacciNumbers.Where(x => x >= start && x <= end);
    
    Console.WriteLine($"Fibonacci numbers between {start} and {end}: {string.Join(", ", results)}");
}

public static void FindTheNFirstFibonacciNumbers(int firstN)
{
    var fibonacciNumbers = new List<int>();
    var fib1 = 0;
    var fib2 = 1;
    fibonacciNumbers.Add(fib1);
    fibonacciNumbers.Add(fib2);

    for (var i = 2; i < firstN; i++)
    {
        var fib3 = fib1 + fib2;
        fibonacciNumbers.Add(fib3);
        fib1 = fib2;
        fib2 = fib3;
    }

    Console.WriteLine($"The first {firstN} Fibonacci numbers are: {string.Join(", ", fibonacciNumbers)}");
}