Answer 1

Recursion means that a function is being called by itself repeatedly, until the base condition is reached. Conceptually it is very similar to a loop. Some problems can be solved with recursion very easily. You can do it in any programming language. There is a lesson about it in the C course.
https://www.sololearn.com/learn/C/2930/

Answer 2

Tibor Santa , recursion can also be indirect when function a calls function b which then callls function a until a base case is reached.
In this case it isnt expicitly calling itself but instead calling a functon that then calls it back.This can happen with more than two functions calling each other.

Answer 3

https://www.sololearn.com/discuss/2112132/?ref=app

Answer 4

An interesting optimization and variation to the recursion logic is "Tail Recursion".

Tail recursion can be implemented more efficiently than general recursion. When we make a normal recursive call, we have to push the return address onto the call stack then jump to the called function. This means that we need a call stack whose size is linear in the depth of the recursive calls.
A recursive function is tail recursive when recursive call is the last thing executed by the function.

Read more on these here:
https://stackoverflow.com/questions/33923/what-is-tail-recursion
https://www.geeksforgeeks.org/tail-recursion/
https://cs.stackexchange.com/questions/6230/what-is-tail-recursion