diff --git a/fibonacci.py b/fibonacci.py
new file mode 100644
index 0000000..eeafe2a
--- /dev/null
+++ b/fibonacci.py
@@ -0,0 +1,94 @@
+def fibonacci(n):
+    """
+    Calculate the nth Fibonacci number.
+    
+    Args:
+        n (int): The position in the Fibonacci sequence (0-indexed)
+    
+    Returns:
+        int: The nth Fibonacci number
+    
+    Raises:
+        ValueError: If n is negative
+    """
+    if n < 0:
+        raise ValueError("n must be a non-negative integer")
+    
+    if n <= 1:
+        return n
+    
+    return fibonacci(n - 1) + fibonacci(n - 2)
+
+
+def fibonacci_iterative(n):
+    """
+    Calculate the nth Fibonacci number using an iterative approach.
+    This is more efficient than the recursive version.
+    
+    Args:
+        n (int): The position in the Fibonacci sequence (0-indexed)
+    
+    Returns:
+        int: The nth Fibonacci number
+    
+    Raises:
+        ValueError: If n is negative
+    """
+    if n < 0:
+        raise ValueError("n must be a non-negative integer")
+    
+    if n <= 1:
+        return n
+    
+    prev, curr = 0, 1
+    for _ in range(2, n + 1):
+        prev, curr = curr, prev + curr
+    
+    return curr
+
+
+def fibonacci_sequence(count):
+    """
+    Generate a list of Fibonacci numbers.
+    
+    Args:
+        count (int): Number of Fibonacci numbers to generate
+    
+    Returns:
+        list: A list containing the first 'count' Fibonacci numbers
+    
+    Raises:
+        ValueError: If count is negative
+    """
+    if count < 0:
+        raise ValueError("count must be a non-negative integer")
+    
+    if count == 0:
+        return []
+    
+    if count == 1:
+        return [0]
+    
+    sequence = [0, 1]
+    for i in range(2, count):
+        sequence.append(sequence[i - 1] + sequence[i - 2])
+    
+    return sequence
+
+
+if __name__ == "__main__":
+    # Example usage
+    print("Fibonacci calculation examples:")
+    print("-" * 40)
+    
+    # Calculate specific Fibonacci numbers
+    for i in range(10):
+        print(f"fibonacci({i}) = {fibonacci(i)}")
+    
+    print("\n" + "-" * 40)
+    print("Using iterative approach:")
+    print(f"fibonacci_iterative(15) = {fibonacci_iterative(15)}")
+    
+    print("\n" + "-" * 40)
+    print("First 15 Fibonacci numbers:")
+    print(fibonacci_sequence(15))