Big-O notation of T(sqrtn) + 5

Question

I face a question: T(N) = T(sqrt(N)) + 5.

I am wondering can I solve it in this way?

T(N) = O(sqrt(N)) + O(5)

Since O(5) = O(1) is a constant, we can ignore it.

So the big O notation of T(N) is O(N^(1/2)).

Or can I just say its notation is O(N) as there is no big difference between O(N) and O(sqrt(N)).

Thank you!

Answer 1

Edit: I made a mistake in the original answer, assuming that n is a power of 2 and reducing the recurrence to 1, 2, 4, ... n, which is wrong. I apologize for the misleading. Here is the updated answer.

From,

T(n) = T(sqrt(n)) + 5,

we can also write it as:

T(n) = T(n^(1/2)) + 5,

then by recurrence:

T(n^(1/2)) = T(n^(1/4)) + 5,

T(n^(1/4)) = T(n^(1/8)) + 5,

...

T(n^(2^-m)) = T(n^(2^-(m+1)) + 5,

this doesn't show a constant where we can stop. Therefore we need to substitute n.

Try:

n = 2^(2^m),

where we have

m = log log n

starting from m = 0, which is n = 2,

then we have:

T(n) = T(2) + 5 + 5 + ... + 5,

how many 5s are there? We count like this:

2^(2^0), 2^(2^1), 2^(2^2), ... 2^(2^m)

So there are m 5s, where m = log log n. So

T(n) = T(2) + 5 log log n,

which is,

T(n) = O(log log n).

Answer 2

(For neatness, let's replace the 5 with a constant c)

Substituting this function into itself multiple times, we can spot a pattern emerging:

When do we stop iterating? When the stopping condition is met. Take this to be n = 2 (not 1 as is usually the case, since the argument is asymptotic to n = 1):

So the final cost of this function is:

Note that the constant c (= 5) does not matter in terms of asymptotic complexity. (And also that the result is not simply log n but log log n)

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EDIT: if you were to choose a different stopping condition n = a, a > 1, then the above step would become:

Which only differs by a constant from the original result.