(The published version and the arXiv version are available online.)
Suppose xm+cxn is a permutation polynomial over Fp, where p > 5 is prime and m > n > 0 and c is a nonzero element of Fp. We prove that gcd(m-n, p-1) is neither 2 nor 4. In the special case that either (p-1)/2 or (p-1)/4 is prime, this had been conjectured by Masuda, Panario and Wang.
Additional comment from July 2007: We have generalized this result in a subsequent paper.
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