Prime Power Conjecture

An Abelian planar Difference Set of order $n$ exists only for $n$ a Prime Power. Gordon (1994) has verified it to be true for $n<2,000,000$.

See also Difference Set

References

Gordon, D. M. ``The Prime Power Conjecture is True for $n<2,000,000$.'' Electronic J. Combinatorics 1, R6 1-7, 1994. http://www.combinatorics.org/Volume_1/volume1.html#R6.