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Turkey TST 1997 Problem 2, how many pairs

Source: Turkey TST 1997 Problem 2

November 30, 2011
algebra proposedalgebra

Problem Statement

The sequences (an)(a_{n}), (bn)(b_{n}) are defined by a1=αa_{1} = \alpha, b1=βb_{1} = \beta, an+1=αanβbna_{n+1} = \alpha a_{n} - \beta b_{n}, bn+1=βan+αbnb_{n+1} = \beta a_{n} + \alpha b_{n} for all n>0.n > 0. How many pairs (α,β)(\alpha, \beta) of real numbers are there such that a1997=b1a_{1997} = b_{1} and b1997=a1b_{1997} = a_{1}?