What are the standard problems we can reduce from to prove $\Omega(n\log n)$ lower bounds?

Of course, state problems other than sorting and element distinctness.

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    $\begingroup$ In what computational model? $\endgroup$ Oct 11, 2011 at 5:28
  • $\begingroup$ Good point. I meant the comparision based model. $\endgroup$ Oct 11, 2011 at 19:39

1 Answer 1


Ben-Or directly proved $\Omega(n\log n)$ lower bounds for several fundamental problems in the algebraic computation tree model:

  • Element distinctness: Given an array of $n$ real numbers, are its elements distinct?
  • Set disjointness: Given two sets of $n$ real numbers, do they have an element in common?
  • Set equality: Given two sets of $n$ real numbers, is one array a permutation of the other?
  • Measure problem: Given $n$ real intervals, what is the total length of their union?
  • Set inclusion: Given two sets of real numbers, is one a subset of the other?
  • Permutation Parity: Given an permutation of the set $[n]$, is the permutation even or odd?

The first three are the ones most often used in computational geometry.

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    $\begingroup$ irrelevant aside: the first three are also the canonical hard problems for communication-complexity-based stream algorithm lower bounds. $\endgroup$ Oct 11, 2011 at 14:01
  • $\begingroup$ @SureshVenkat - I have seen set disjointness and set equality being used for proving lower bounds in streaming. Do you have an example for element distinctness? $\endgroup$ Oct 11, 2011 at 19:42
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    $\begingroup$ At least one place I saw it was in the analysis of algorithms under the W-stream model. In general, ED is closely related to bit-vector (or set) disjointness $\endgroup$ Oct 11, 2011 at 20:14

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