Let be a sequence of positive integers such that for integers . How many such sequences are there such that ?
Solution
Consider the characteristic polynomial for the recurrence , which is . The roots are at 2 and 1 , so we know that numbers must be of the form for integers and . Therefore must equal to , where and are both integers. If the expression is always positive, it is sufficient to say is positive and is nonnegative, or , and . For a given value of , so there are possible values of for each (where the quantity is positive). can take any value between 0 and , we sum over all such a in this range, to attain , or , which is our answer.
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