Final answer:
The question deals with implementing a function to return the nth value of the Fibonacci sequence, where the sequence begins with 0 and 1, with each subsequent number being the sum of the two preceding numbers.
Step-by-step explanation:
The subject of this question is the Fibonacci sequence, which is a series of numbers where each number is the sum of the two preceding ones, usually starting with 0 and 1. The task is to complete a function that returns the nth value in the sequence. For negative index values, the function should return -1. This is a mathematical concept that has intriguing connections to various natural phenomena, including the patterns seen in spirals of plant growth, which can also be explored via resources like Vihart's video on spirals and the Fibonacci series or the MoMath's explanations of how to count spirals using the Fibonacci series.
Example Fibonacci Function in Python
Here is a simple implementation of a Fibonacci function:
def fibonacci(n):
if n < 0:
return -1
elif n == 0:
return 0
elif n == 1:
return 1
else:
a, b = 0, 1
for _ in range(2, n + 1):
a, b = b, a + b
return b
This function works by iterating through the sequence, starting with the 0 and 1, and summing the previous two numbers to get the next one until it reaches the required index position (n).