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Ukrajina 2008

Ukraine 2008 algebra

Problem

Sequence is defined as follows: , , for random natural . What may be the greatest number of members of the sequence among the consecutive members of the increasing arithmetic progression?
Solution
Let the first consecutive members be , . Since , , , , the third member may only be .

So . It's clear that the next member of progression does not satisfy the condition because . But we can easily find the three consecutive members 1, 2, 3, and 2, 5, 8.
Final answer
3

Techniques

Recurrence relationsLinear and quadratic inequalities