The positive integers $N$ and $N^2$ both end in the same sequence of four digits $abcd$ when written in base 10, where digit $a$ is not zero. Find the three-digit number $abc$.
1. Given that both \( N \) and \( N^2 \) end in the same sequence of four digits \( abcd \), we can express this condition mathematically as:
   \[
   N \equiv N^2 \pmod{10000}
   \]
   This implies:
   \[
   N^2 - N \equiv 0 \pmod{10000}
   \]
   Therefore:
   \[
   N(N-1) \equiv 0 \pmod{10000}
   \]

2. To satisfy \( N(N-1) \equiv 0 \pmod{10000} \), the product \( N(N-1) \) must be divisible by \( 10000 \). We can factorize \( 10000 \) as:
   \[
   10000 = 2^4 \times 5^4
   \]
   Hence, \( N \) and \( N-1 \) must collectively account for the factors \( 2^4 \) and \( 5^4 \).

3. Since \( N \) and \( N-1 \) are consecutive integers, one of them must be divisible by \( 16 \) (i.e., \( 2^4 \)) and the other by \( 625 \) (i.e., \( 5^4 \)).

4. We consider two cases:
   - Case 1: \( N \) is a multiple of \( 625 \)
   - Case 2: \( N \) is a multiple of \( 16 \)

5. **Case 1: \( N \) is a multiple of \( 625 \)**
   - Let \( N = 625k \). Then \( N-1 = 625k - 1 \).
   - For \( N-1 \) to be divisible by \( 16 \), we need:
     \[
     625k - 1 \equiv 0 \pmod{16}
     \]
     Simplifying, we get:
     \[
     625k \equiv 1 \pmod{16}
     \]
     Since \( 625 \equiv 1 \pmod{16} \), we have:
     \[
     k \equiv 1 \pmod{16}
     \]
     Therefore, \( k = 16m + 1 \) for some integer \( m \).

6. Substituting \( k = 16m + 1 \) into \( N = 625k \), we get:
   \[
   N = 625(16m + 1) = 10000m + 625
   \]
   Since \( N \) must be a four-digit number, \( m = 0 \) is the only possible value, giving:
   \[
   N = 625
   \]
   However, \( 625 \) is not a four-digit number, so this case is invalid.

7. **Case 2: \( N \) is a multiple of \( 16 \)**
   - Let \( N = 16k \). Then \( N-1 = 16k - 1 \).
   - For \( N-1 \) to be divisible by \( 625 \), we need:
     \[
     16k - 1 \equiv 0 \pmod{625}
     \]
     Simplifying, we get:
     \[
     16k \equiv 1 \pmod{625}
     \]
     We need to find the multiplicative inverse of \( 16 \) modulo \( 625 \).

8. Using the Extended Euclidean Algorithm to find the inverse of \( 16 \) modulo \( 625 \):
   - \( 625 = 39 \times 16 + 1 \)
   - \( 1 = 625 - 39 \times 16 \)
   - Thus, the inverse of \( 16 \) modulo \( 625 \) is \( -39 \equiv 586 \pmod{625} \).

9. Therefore:
   \[
   k \equiv 586 \pmod{625}
   \]
   Hence, \( k = 625m + 586 \) for some integer \( m \).

10. Substituting \( k = 625m + 586 \) into \( N = 16k \), we get:
    \[
    N = 16(625m + 586) = 10000m + 9376
    \]
    Since \( N \) must be a four-digit number, \( m = 0 \) is the only possible value, giving:
    \[
    N = 9376
    \]

11. The three-digit number \( abc \) is the first three digits of \( 9376 \), which is \( 937 \).

The final answer is \( \boxed{937} \).