A5 For a nonnegative integer k, let f(k) be the number of ones in the base 3 representation of k. Find all complex numbers z such thatk=0∑31010−1(−2)f(k)(z+k)2023=0
Q. A5 For a nonnegative integer k, let f(k) be the number of ones in the base 3 representation of k. Find all complex numbers z such thatk=0∑31010−1(−2)f(k)(z+k)2023=0
Understand function f(k): To solve this problem, we need to understand the nature of the function f(k) and how it affects the sum. The function f(k) counts the number of ones in the base 3 representation of k. This means that for each k, the exponent of −2 will be determined by how many ones are in the base 3 representation of k. We need to consider the properties of the sum and the terms involved.
Consider sum of terms: Let's consider the sum in question. We have a sum of terms of the form (−2)f(k)(z+k)2023 from k=0 to 31010−1. Notice that the sum is over all nonnegative integers k less than 31010. This range includes all possible base 3 representations with 1010 digits (including leading zeros).
Cancellation of terms: The sum is equal to zero only if each term in the sum is zero or if the terms cancel each other out. Since (z+k)2023 is raised to an odd power, it will not be zero unless z=−k. However, z cannot be equal to −k for all k in the range simultaneously. Therefore, we must rely on the cancellation of terms.
Cycle of base 3 representations: We need to consider the properties of the exponents of −2, which are determined by f(k). Since the base 3 representation of k determines f(k), we can see that f(k) will cycle through values as k increases. For example, f(0)=0, f(1)=1, f(2)=0, f(k)0 (since 3 in base 3 is f(k)3), and so on.
Cancellation over complete cycle: The key insight is that the sum is over a complete cycle of base 3 representations from 0 to 31010−1. This means that for every possible pattern of ones in the base 3 representation, there is an equal number of k's with that pattern. Therefore, the terms (−2)f(k) will cancel out over the complete cycle since for every f(k)=n, there will be an equal number of f(k)=n terms with a positive and negative sign.
Simplify sum to find z: Given that the terms (−2)f(k) cancel out over the complete cycle, the sum simplifies to the sum of (z+k)2023 from k=0 to 31010−1. Since we are looking for values of z such that this sum equals zero, we need to consider the possibility that z is a complex number that causes these terms to cancel out.
Complex number consideration: However, since (z+k)2023 is a high-degree polynomial in k, and the sum is over a symmetric range of k values, there is no obvious reason why these terms should cancel out for any particular value of z. In fact, for any given z, the terms (z+k)2023 will generally not cancel out, as they will have different magnitudes and phases when z is complex.
Terms do not cancel out: Therefore, we conclude that there is no value of z that will satisfy the given equation. The sum cannot be zero for any complex number z because the terms (z+k)2023 do not cancel out in the sum from k=0 to 31010−1.