How does one write a recursive function for Fibonacci sequence?

A recursive function for the Fibonacci sequence can be written by calling the function within itself to calculate the sequence.

The Fibonacci sequence is a series of numbers where a number is the sum of the two preceding ones, usually starting with 0 and 1. In mathematical terms, the sequence is defined by the recurrence relation: F(n) = F(n-1) + F(n-2), with seed values F(0) = 0 and F(1) = 1.

To write a recursive function for the Fibonacci sequence in a programming language, you would first define the base cases for the function. The base cases are the conditions under which the function stops calling itself and returns a value. For the Fibonacci sequence, the base cases are F(0) = 0 and F(1) = 1. In code, this could look something like this:

```
if n == 0:
return 0
elif n == 1:
return 1
```

Next, you would write the recursive case for the function. The recursive case is where the function calls itself with a different argument. For the Fibonacci sequence, the recursive case is F(n) = F(n-1) + F(n-2). In code, this could look something like this:

```
else:
return fibonacci(n-1) + fibonacci(n-2)
```

Putting it all together, a recursive function for the Fibonacci sequence could look something like this:

```
def fibonacci(n):
if n == 0:
return 0
elif n == 1:
return 1
else:
return fibonacci(n-1) + fibonacci(n-2)
```

This function works by repeatedly calling itself with smaller and smaller values of n until it reaches the base cases, at which point it starts returning values and adding them together to calculate the Fibonacci number for the original value of n.

Remember, recursion can be a difficult concept to grasp at first, but with practice, it becomes a powerful tool in your programming arsenal.