Consider the sums of the form $\sum_{k=1}^{n} \epsilon_k k^3,$ where $\epsilon_k \in \{-1, 1\}.$ Is any of these sums equal to $0$ if

[b](a)[/b] $n=2000;$

[b](b)[/b] $n=2001 \ ?$
Let's analyze the problem step by step for both parts (a) and (b).

### Part (a): \( n = 2000 \)

1. **Understanding the Problem:**
   We need to determine if there exists a sequence of \(\epsilon_k \in \{-1, 1\}\) such that the sum \(\sum_{k=1}^{n} \epsilon_k k^3 = 0\) for \(n = 2000\).

2. **Exploring the Parity:**
   Notice that the sum of cubes of the first \(n\) natural numbers is given by:
   \[
   \sum_{k=1}^{n} k^3 = \left( \frac{n(n+1)}{2} \right)^2
   \]
   For \(n = 2000\), we have:
   \[
   \sum_{k=1}^{2000} k^3 = \left( \frac{2000 \cdot 2001}{2} \right)^2 = 1000^2 \cdot 2001^2
   \]
   This is an even number since it is a product of even numbers.

3. **Constructing the Sum:**
   We need to find a combination of \(\epsilon_k\) such that the sum of the cubes with these signs equals zero. We can use the fact that the sum of cubes can be partitioned into smaller sums that cancel each other out.

4. **Using Symmetry:**
   Consider the function \(f(n) = (n+1)^3 - n^3\):
   \[
   f(n) = (n+1)^3 - n^3 = 3n^2 + 3n + 1
   \]
   This function represents the difference between consecutive cubes. We can use this to construct sums that cancel out.

5. **Finding Patterns:**
   We can look for patterns in the differences:
   \[
   g(n) = f(n+2) - f(n) = 12n + 18
   \]
   \[
   h(n) = g(n+4) - g(n) = 48
   \]
   \[
   h(n+8) - h(n) = 0
   \]
   This shows that we can create sums of cubes that cancel out in blocks of 16 consecutive terms.

6. **Conclusion for \(n = 2000\):**
   Since \(2000\) is divisible by \(16\), we can partition the sequence into blocks of \(16\) terms each, and within each block, we can arrange the \(\epsilon_k\) such that the sum of cubes is zero.

\[
\boxed{\text{True}}
\]

### Part (b): \( n = 2001 \)

1. **Understanding the Problem:**
   We need to determine if there exists a sequence of \(\epsilon_k \in \{-1, 1\}\) such that the sum \(\sum_{k=1}^{n} \epsilon_k k^3 = 0\) for \(n = 2001\).

2. **Exploring the Parity:**
   Notice that the sum of cubes of the first \(n\) natural numbers is given by:
   \[
   \sum_{k=1}^{2001} k^3 = \left( \frac{2001 \cdot 2002}{2} \right)^2
   \]
   This is an odd number since it is a product of an odd number and an even number.

3. **Parity Argument:**
   Since the sum of cubes for \(n = 2001\) is odd, and each \(\epsilon_k k^3\) is either \(k^3\) or \(-k^3\), the sum \(\sum_{k=1}^{2001} \epsilon_k k^3\) must also be odd. Therefore, it cannot be zero, as zero is an even number.

\[
\boxed{\text{False}}
\]