(The published version and the arXiv version are available online.)

Suppose  x^m+cxⁿ  is a permutation polynomial over  F_p,  where  p > 5  is prime and  m > n > 0  and  c  is a nonzero element of  F_p.  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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