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Algebra Difficulty 5.2 AIME, harder Prove it United States

Problem:

Let a1,a2,a_{1}, a_{2}, \ldots be an infinite sequence of positive integers such that for integers n>2n > 2, an=3an12an2a_{n} = 3 a_{n-1} - 2 a_{n-2}. How many such sequences {an}\{a_{n}\} are there such that a201022012a_{2010} \leq 2^{2012}?

Solution

Solution:

Answer: 3622009+3636 \cdot 2^{2009} + 36

Consider the characteristic polynomial for the recurrence an+23an+1+2an=0a_{n+2} - 3 a_{n+1} + 2 a_{n} = 0, which is x23x+2x^{2} - 3x + 2. The roots are at 22 and 11, so we know that numbers aia_{i} must be of the form a2i1+ba 2^{i-1} + b for integers aa and bb. Therefore a2010a_{2010} must equal a22009+ba 2^{2009} + b, where aa and bb are both integers. If the expression is always positive, it is sufficient to say a1a_{1} is positive and aa is nonnegative, or a+b>0a + b > 0, and a0a \geq 0.

For a given value of aa, 1ab22012a220091 - a \leq b \leq 2^{2012} - a 2^{2009}, so there are 22012a22009+a2^{2012} - a 2^{2009} + a possible values of bb for each aa (where the quantity is positive). aa can take any value between 00 and 232^{3}, we sum over all such aa in this range, to attain 922012(1+2+3+4+5+6+7+8)22009+(1+2+3+4+5+6+7+8)9 \cdot 2^{2012} - (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8) 2^{2009} + (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8), or 36(22009)+3636\left(2^{2009}\right) + 36, which is our answer.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.