On periodic solutions of 2-periodic Lyness difference equations

Agraïments: CoDALab group is supported by the Catalonia's government through the SGR program. The support of DMA3's Terrassa Campus Section is also acknowledged. We study the existence of periodic solutions of the non-autonomous periodic Lyness'recurrence un+2 = (an + un+1)/un, where {an}n is a cycle with positive values a,b and with positive initial conditions. It is known that for a = b = 1 all the sequences generated by this recurrence are 5-periodic. We prove that for each pair (a, b) 6= (1, 1) there are infinitely many initial conditions giving rise to periodic sequences, and that the family of recurrences have almost all the even periods. If a 6= b, then any odd period, except 1, appears