Theory of Computing
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Title : Inverting a Permutation is as Hard as Unordered Search
Authors : Ashwin Nayak
Volume : 7
Number : 2
Pages : 19-25
URL : http://www.theoryofcomputing.org/articles/v007a002
Abstract
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We show how an algorithm for the problem of inverting a permutation
may be used to design one for the problem of unordered search (with
a unique solution). Since there is a straightforward reduction in
the reverse direction, the problems are essentially equivalent.
The reduction we present helps us bypass the hybrid argument due to
Bennett, Bernstein, Brassard, and Vazirani (1997) and the quantum
adversary method due to Ambainis (2002) that were earlier used to
derive lower bounds on the quantum query complexity of the problem
of inverting permutations. It directly implies that the quantum
query complexity of the problem is asymptotically the same as that
for unordered search, namely in \Theta(\sqrt{n}).