Version 6 changes:
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-- More or less the final journal version. (2/5/2014)
-- Fixed minor typos. (2/5/2014)
-- Added Remark 23 explaining connection to sparse oblivious subspace
   embeddings.
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Version 5 changes:
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-- Modified the abstract and fixed some typos. (8/21/2013)
-- Added an open problems section at end of paper. (8/21/2013)
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Version 4 changes:
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-- Simplified section 4 by giving 1 analysis that covers both
   constructions. Also fixed some minor typos. (12/7/2012)
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Version 3 changes:
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-- Fixed some errors in the version 2 writeup of the proof of Theorem
   25.  The theorem conclusion is unchanged.  In particular, the v2
   proof took q = Theta(log(1/delta)/log(1/eps)) in the case s =
   Omega(1/eps) and looked at q items colliding.  This makes no sense
   if q > s (which can happen depending on the relationship between
   eps, delta). Really one should take q = Theta(sqrt(eps*s)), which
   is Theta(log(1/delta)/log(1/eps)) for the largest setting of s, but
   not always.  See the version 3 paper for details. (4/28/2011)
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Version 2 changes:
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-- Showed that the construction of [Dasgupta-Kumar-Sarlos, STOC 2010]
   requires sparsity Omega~(eps^{-1} * log^2(1/delta)). (4/14/2011)
-- Gave a second new construction which achieves sparsity Theta(eps^{-1} *
   log(1/delta)). (4/14/2011)
-- Changed the title to "Sparser Johnson-Lindenstrauss Transforms", since
   we now give two constructions. (4/14/2011)