  In 1985, Robbins observed by computer the continued fraction expansion of
certain algebraic power series over a finite field. Incidentally, he came
across a particular equation of degree 4 in characteristic p=13. This equation
has an analogue for all primes p>=5. There are two patterns for the continued
fraction of the solution of this equation, according to the residue of p modulo
3. We describe this pattern in the first case, considering especially p=7 and
p=13. in the second case we only give indications.
