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Abstract for Daniel M. Gordon, The prime power conjecture is true for 
$n < 2{,}000{,}000$.

The Prime Power Conjecture (PPC) states that abelian planar
difference sets of order $n$ exist only for $n$ a prime power.
Evans and Mann verified this for cyclic difference sets for
$n \leq 1600$.  In this paper we verify the PPC
for $n \leq 2{,}000{,}000$, using many necessary conditions on the group
of multipliers.
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