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    Here is the formal definition of Big-O: f(n) = O(g(n)) means there are positive constants c and k, such that 0 ≤ f(n) ≤ cg(n) for all n ≥ k. So,saying that there is no such thing as O(2xN) is wrong. Although, you may say that O(N)=O(2xN).