Problem:
Let be an infinite sequence of positive integers such that for integers , . How many such sequences are there such that ?
Problem:
Let be an infinite sequence of positive integers such that for integers , . How many such sequences are there such that ?
Solution:
Answer:
Consider the characteristic polynomial for the recurrence , which is . The roots are at and , so we know that numbers must be of the form for integers and . Therefore must equal , 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 and , we sum over all such in this range, to attain , or , which is our answer.