neural

Here's a Python function that calculates the Fibonacci sequence up to a given number n, using an efficient iterative approach:

```python
def fibonacci(n):
    if n <= 0:
        return []
    elif n == 1:
        return [0]
    fib_sequence = [0, 1]
    while len(fib_sequence) < n:
        next_value = fib_sequence[-1] + fib_sequence[-2]
        fib_sequence.append(next_value)
    return fib_sequence
```

This function first handles edge cases for n <= 0 and n == 1. For larger values of n, it initializes the sequence with the first two Fibonacci numbers and then iteratively calculates subsequent values by summing the last two numbers in the sequence. The function returns the sequence as a list.