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def fibonacci(n):
    if n <= 1: # Base condition
        return n
    else:
        # The nth Fibonacci number = (n-1)th Fibonacci number + (n-2)th Fibonacci number
        return fibonacci(n-1) + fibonacci(n-2)

print('fibonacci(10):', fibonacci(10))

# Implementing Fibonacci Sequence with Recursive Function

The Fibonacci sequence is a series of numbers in which each number is the sum of the two preceding ones.

It typically starts with 0 and 1, progressing as 0, 1, 1, 2, 3, 5, 8, 13, 21, ...

The Fibonacci numbers can be mathematically defined as F(n) = F(n-1) + F(n-2), and can be implemented in Python as follows:

```python title="Fibonacci Sequence Implementation"
def fibonacci(n):
    if n <= 1:
        return n
    else:
        return fibonacci(n-1) + fibonacci(n-2)
```

<br />

## Time Complexity

The time complexity of implementing the Fibonacci sequence using a recursive function is O(2^n). This is because the function makes two function calls at each step, leading to exponential growth in the number of calls.