Find (in terms of $n \ge 1$) the number of terms with odd coefficients after expanding the product:
\[\prod_{1 \le i < j \le n} (x_i + x_j)\]
e.g., for $n = 3$ the expanded product is given by $x_1^2 x_2 + x_1^2 x_3 + x_2^2 x_3 + x_2^2 x_1 + x_3^2 x_1 + x_3^2 x_2 + 2x_1 x_2 x_3$ and so the answer would be $6$.
1. **Understanding the Problem:**
   We need to find the number of terms with odd coefficients in the expansion of the product:
   \[
   \prod_{1 \le i < j \le n} (x_i + x_j)
   \]
   For example, for \( n = 3 \), the expanded product is:
   \[
   x_1^2 x_2 + x_1^2 x_3 + x_2^2 x_3 + x_2^2 x_1 + x_3^2 x_1 + x_3^2 x_2 + 2x_1 x_2 x_3
   \]
   Here, the terms with odd coefficients are \( x_1^2 x_2, x_1^2 x_3, x_2^2 x_3, x_2^2 x_1, x_3^2 x_1, x_3^2 x_2 \), which total to 6 terms.

2. **First Proof:**
   - Suppose \( a_i = a_j \) for some \( i \neq j \). We claim that the coefficient of \( x_1^{a_1} x_2^{a_2} \dots x_n^{a_n} \) is even.
   - Consider the factor \( x_i + x_j \) in the product. By symmetry, the number of ways to get \( x_1^{a_1} x_2^{a_2} \dots x_n^{a_n} \) by choosing the \( x_i \) term is equal to the number of ways to get that while choosing the \( x_j \) term.
   - Thus, there are an even number of ways to get \( x_1^{a_1} x_2^{a_2} \dots x_n^{a_n} \), so it has an even coefficient.

3. **Distinct Exponents:**
   - Now, suppose that \( a_1, a_2, \dots, a_n \) are pairwise distinct. Since they are nonnegative integers summing to \( \binom{n}{2} \), we see that \( \{a_1, a_2, \dots, a_n\} = \{0, 1, \dots, n - 1\} \).
   - We will show that there is exactly one way to achieve \( x_1^{a_1} x_2^{a_2} \dots x_n^{a_n} \). Without loss of generality, assume that \( a_1 = 0, a_2 = 1, \dots, a_n = n - 1 \).
   - To get an \( x_n^{n - 1} \) term, we need to choose \( x_n \) from each of the factors \( x_1 + x_n, x_2 + x_n, \dots, x_{n - 1} + x_n \). Proceeding in a similar fashion, we see that every variable is forced.
   - Thus, the coefficient of \( x_1^{a_1} x_2^{a_2} \dots x_n^{a_n} \) is odd if and only if \( \{a_1, a_2, \dots, a_n\} = \{0, 1, \dots, n - 1\} \).

4. **Second Proof:**
   - We can change the product to:
     \[
     \prod_{1 \le i < j \le n} (x_j - x_i)
     \]
     without changing the answer, since we only care about the coefficients modulo 2.
   - This is a Vandermonde determinant, so by a property of Vandermonde determinants, this has exactly \( n! \) terms in its expansion, each of which is of the form \( \pm x_1^{a_1} x_2^{a_2} \dots x_n^{a_n} \) for each \( \{a_1, a_2, \dots, a_n\} = \{0, 1, \dots, n - 1\} \).
   - Thus, the coefficient of \( x_1^{a_1} x_2^{a_2} \dots x_n^{a_n} \) is odd if and only if \( \{a_1, a_2, \dots, a_n\} = \{0, 1, \dots, n - 1\} \).

5. **Conclusion:**
   - There are \( n! \) terms of this type, so the answer is \( n! \).

The final answer is \( \boxed{ n! } \)