Assume that \(𝑓(𝑛)\) and \(𝑔(𝑛)\) are asymptotically positive. Which of…

2022

Assume thatΒ \(𝑓(𝑛)\)Β andΒ \(𝑔(𝑛)\)Β are asymptotically positive. Which of the following is correct ?

  1. A.

    \(f(n)=O(g(n))\)Β andΒ \(g(n)=O(h(n)) \Rightarrow f(n)=\omega(h(n))\)

  2. B.

    \(f(n)=\Omega(g(n))\)Β andΒ \(g(n)=\Omega(h(n)) \Rightarrow f(n)=O(h(n))\)

  3. C.

    \(f(n)=o(g(n))\)Β andΒ \(g(n)=o(h(n)) \Rightarrow f(n)=o(h(n))\)

  4. D.

    \(f(n)=\omega(g(n))\)Β andΒ \(g(n)=\omega(h(n)) \Rightarrow f(n)=\Omega(h(n))\)

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Correct answer: C

Summary: determine which implications are true and give short justifications.

  • Statement "f(n)=O(g(n)) and g(n)=O(h(n)) β‡’ f(n)=Ο‰(h(n))": false. Counterexample: take f(n)=n, g(n)=n, h(n)=n. The premises hold but f(n)/h(n)=1, so f(n) is not Ο‰(h(n)). The correct conclusion is f(n)=O(h(n)).

  • Statement "f(n)=Ξ©(g(n)) and g(n)=Ξ©(h(n)) β‡’ f(n)=O(h(n))": false. From the premises you get f(n)=Ξ©(h(n)). Example: f(n)=n^2, g(n)=n, h(n)=1 satisfy the premises, but f(n) is not O(h(n)).

  • Statement "f(n)=o(g(n)) and g(n)=o(h(n)) β‡’ f(n)=o(h(n))": true. Proof sketch: f(n)/h(n) = (f(n)/g(n))Β·(g(n)/h(n)), and each factor tends to 0, so the product tends to 0.

  • Statement "f(n)=Ο‰(g(n)) and g(n)=Ο‰(h(n)) β‡’ f(n)=Ξ©(h(n))": true (in fact stronger). Since f(n)/g(n)β†’βˆž and g(n)/h(n)β†’βˆž, their product f(n)/h(n)β†’βˆž, so f(n)=Ο‰(h(n)), which implies f(n)=Ξ©(h(n)).

Final verdict: The small-o transitivity statement and the omega-based statement are both true. Therefore both of the statements that assert small-o transitivity and that omega-transitivity implies an Omega relation are correct.

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