n log n is O(n)?

Question

I am trying to solve this recurrence

T(n) = 3 T(n/2) + n lg n ..

I have come to the solution that it belongs to masters theorem case 2 since n lg n is O(n^2

Answer 1

n*log(n) is not O(n^2). It's known as quasi-linear and it grows much slower than O(n^2). In fact n*log(n) is less than polynomial.

In other words:

O(n*log(n)) < O(n^k)

where k > 1

In your example:

3*T(2n) -> O(n^1.585)

Since O(n^1.585) is polynomial and dominates O(n*log(n)), the latter term drops off so the final complexity is just O(n^1.585).