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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 that

sum_(k=0)^(3^(1010)-1)(-2)^(f(k))(z+k)^(2023)=0

A55 For a nonnegative integer k k , let f(k) f(k) be the number of ones in the base 33 representation of k k . Find all complex numbers z z such that\newlinek=0310101(2)f(k)(z+k)2023=0 \sum_{k=0}^{3^{1010}-1}(-2)^{f(k)}(z+k)^{2023}=0

Full solution

Q. A55 For a nonnegative integer k k , let f(k) f(k) be the number of ones in the base 33 representation of k k . Find all complex numbers z z such that\newlinek=0310101(2)f(k)(z+k)2023=0 \sum_{k=0}^{3^{1010}-1}(-2)^{f(k)}(z+k)^{2023}=0
  1. Understand function f(k)f(k): To solve this problem, we need to understand the nature of the function f(k)f(k) and how it affects the sum. The function f(k)f(k) counts the number of ones in the base 33 representation of kk. This means that for each kk, the exponent of 2-2 will be determined by how many ones are in the base 33 representation of kk. We need to consider the properties of the sum and the terms involved.
  2. 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(-2)^{f(k)}(z+k)^{2023} from k=0k=0 to 3101013^{1010}-1. Notice that the sum is over all nonnegative integers kk less than 310103^{1010}. This range includes all possible base 33 representations with 10101010 digits (including leading zeros).
  3. 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(z+k)^{2023} is raised to an odd power, it will not be zero unless z=kz = -k. However, zz cannot be equal to k-k for all kk in the range simultaneously. Therefore, we must rely on the cancellation of terms.
  4. Cycle of base 33 representations: We need to consider the properties of the exponents of 2-2, which are determined by f(k)f(k). Since the base 33 representation of kk determines f(k)f(k), we can see that f(k)f(k) will cycle through values as kk increases. For example, f(0)=0f(0) = 0, f(1)=1f(1) = 1, f(2)=0f(2) = 0, f(k)f(k)00 (since 33 in base 33 is f(k)f(k)33), and so on.
  5. Cancellation over complete cycle: The key insight is that the sum is over a complete cycle of base 33 representations from 00 to 3101013^{1010}-1. This means that for every possible pattern of ones in the base 33 representation, there is an equal number of kk's with that pattern. Therefore, the terms (2)f(k)(-2)^{f(k)} will cancel out over the complete cycle since for every f(k)=nf(k) = n, there will be an equal number of f(k)=nf(k) = n terms with a positive and negative sign.
  6. Simplify sum to find zz: Given that the terms (2)f(k)(-2)^{f(k)} cancel out over the complete cycle, the sum simplifies to the sum of (z+k)2023(z+k)^{2023} from k=0k=0 to 3101013^{1010}-1. Since we are looking for values of zz such that this sum equals zero, we need to consider the possibility that zz is a complex number that causes these terms to cancel out.
  7. Complex number consideration: However, since (z+k)2023(z+k)^{2023} is a high-degree polynomial in kk, and the sum is over a symmetric range of kk values, there is no obvious reason why these terms should cancel out for any particular value of zz. In fact, for any given zz, the terms (z+k)2023(z+k)^{2023} will generally not cancel out, as they will have different magnitudes and phases when zz is complex.
  8. Terms do not cancel out: Therefore, we conclude that there is no value of zz that will satisfy the given equation. The sum cannot be zero for any complex number zz because the terms (z+k)2023(z+k)^{2023} do not cancel out in the sum from k=0k=0 to 3101013^{1010}-1.

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