  Voting theory has become increasingly integrated with computational social
choice and multiagent systems. Computational complexity has been extensively
used as a shield against manipulation of voting systems, however for several
voting schemes this complexity may cause calculating the winner to be
computationally difficult. Of the many voting systems that have been studied
with regard to election manipulation, a few have been found to have an
unweighted coalitional manipulation problem that is NP-hard for a constant
number of manipulators despite having a winner problem that is in P. We survey
this interesting class of voting systems and the work that has analyzed their
complexity.
