Determine all 6-digit numbers $(abcdef)$ so that $(abcdef) = (def)^2$ where $\left( x_1x_2...x_n \right)$ is no multiplication but an n-digit number.
To determine all 6-digit numbers $(abcdef)$ such that $(abcdef) = (def)^2$, we need to follow these steps:

1. **Identify the range of $(def)$:**
   Since $(abcdef)$ is a 6-digit number, $(def)$ must be a 3-digit number. Let $(def) = n$, where $100 \leq n \leq 999$.

2. **Identify possible values for $f$:**
   The last digit of $n^2$ must be the same as the last digit of $n$. The digits for which their square ends in the same digit are 0, 1, 5, and 6. Therefore, $f$ can be 0, 1, 5, or 6.

3. **Case $f = 0$:**
   - If $f = 0$, then $n$ ends in 0. Therefore, $n$ must be a multiple of 10.
   - The square of any number ending in 0 ends in 00, which means $e$ and $d$ must also be 0.
   - Thus, $(def) = 000$, which is not a 3-digit number. Hence, this case is invalid.

4. **Case $f = 1$:**
   - If $f = 1$, then $n$ ends in 1.
   - Considering the tens digit of $n^2$, we have $2e \equiv e \pmod{10} \Rightarrow e = 0$.
   - Considering the hundreds digit of $n^2$, we have $2d \equiv d \pmod{10} \Rightarrow d = 0$.
   - Thus, $(def) = 001$, which is not a 3-digit number. Hence, this case is invalid.

5. **Case $f = 5$:**
   - If $f = 5$, then $n$ ends in 5.
   - Considering the tens digit of $n^2$, we have $5e + 2 \equiv e \pmod{10} \Rightarrow 4e \equiv 8 \pmod{10} \Rightarrow e = 2$.
   - Considering the hundreds digit of $n^2$, we have $5d + 1 + 5 \equiv d \pmod{10} \Rightarrow 5d + 6 \equiv d \pmod{10} \Rightarrow 4d \equiv 4 \pmod{10} \Rightarrow d = 6$.
   - Thus, $(def) = 625$ and $(abcdef) = 625^2 = 390625$.

6. **Case $f = 6:**
   - If $f = 6$, then $n$ ends in 6.
   - Considering the tens digit of $n^2$, we have $6e + 3 \equiv e \pmod{10} \Rightarrow 5e \equiv 7 \pmod{10} \Rightarrow e = 7$.
   - Considering the hundreds digit of $n^2$, we have $6d + 4 + 3 \equiv d \pmod{10} \Rightarrow 6d + 7 \equiv d \pmod{10} \Rightarrow 5d \equiv 3 \pmod{10} \Rightarrow d = 3$.
   - Thus, $(def) = 376$ and $(abcdef) = 376^2 = 141376$.

Thus, the only two such integers $(abcdef)$ are $390625$ and $141376$.

The final answer is $\boxed{390625}$ and $\boxed{141376}$.