On 12/02/2014 04:25, William Elliot wrote: >> Let a=(1+isqrt(7))/2 and a_n=the real part of a^n >> question : show lim |a_n | is +inf ? > > That's not a question. That's an exercise. > > Let r = sqr 7, b = 1 + ir. > > Below I show how to prove the limit is infinite for 1 + i.sqr 7.