Let f(n), g(n) and h(n) be functions defined for positive inter such that f(n)…

2004

Let f(n), g(n) and h(n) be functions defined for positive inter such that
f(n) = O(g(n)), g(n) ≠ O(f(n)), g(n) = O(h(n)), and h(n) = O(g(n)).

Which one of the following statements is FALSE?

  1. A.

    f(n) + g(n) = O(h(n)) + h(n))

  2. B.

    f(n) = O(h(n))

  3. C.

    fh(n) ≠ O(f(n))

  4. D.

    f(n)h(n) ≠ O(g(n)h(n))

Attempted by 177 students.

Show answer & explanation

Correct answer: D

Given:

  • f(n) = O(g(n))

  • g(n) ≠ O(f(n)) (so g grows strictly faster than f in the asymptotic sense)

  • g(n) = O(h(n)) and h(n) = O(g(n)) (so g and h are Theta of each other)

Consequences and reasoning:

  • By transitivity of O(), from f(n) = O(g(n)) and g(n) = O(h(n)) we get f(n) = O(h(n)).

  • Therefore f(n) + g(n) = O(g(n)) and since g(n) = O(h(n)), we have f(n) + g(n) = O(h(n)).

  • Multiplying the inequality f(n) ≤ C·g(n) by h(n) ≥ 0 yields f(n)h(n) ≤ C·g(n)h(n), so f(n)h(n) = O(g(n)h(n)).

  • If f(n)h(n) were O(f(n)), then for large n with f(n)>0 we would have h(n) = O(1). But since h(n) = Theta(g(n)) and g(n) is not O(f(n)), h(n) cannot be bounded by a constant; hence f(n)h(n) is not O(f(n)).

Conclusion: The statement claiming that f(n)h(n) is not O(g(n)h(n)) is false because f(n)h(n) = O(g(n)h(n)). The other statements (f(n) = O(h(n)), f(n) + g(n) = O(h(n)), and f(n)h(n) ≠ O(f(n))) are true under the given hypotheses.

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