Recurrence Equation Question I have some difficulty trying to tell which equation to use when I'm given an explanation on how an algorithm operates. Especially divide and conquer.

Normally I see these kind of equations:

C(n) = aC(n/a) + b where a and b are constants


Other times I don't see the a in front of C(n/a) as an answer. That really confuses me.

Can you tell me when I will need to use which?

Thanks!
To use any formula, you need to think about what it means. You can't just blindly apply formulas.

$C(n) = aC(n/b) + c$ (and, yes, I've changed the constants) means that the cost of running the algorithm on an input of size&nbsp;$n$ is $a$&nbsp;times the cost of running it on an input of size $n/b$, plus some constant overhead&nbsp;$c$.  A typical case of that would be an algorithm that chops its input into $b$&nbsp;equally sized parts and makes recursive calls on $a$ of those parts. Normally, you'd have $a=b$ (e.g., split the input into three and recurse on each part), which is why the formula you quoted has just two constants.  But you could imagine some sort of average-case analysis where you split into some number of parts and, on average, only need to look at half of them.