Observe that the fraction $ \frac{1}{7}\equal{}0,142857$ is a pure periodical decimal with period $ 6\equal{}7\minus{}1$,and in one period one has $ 142\plus{}857\equal{}999$.For $ n\equal{}1,2,\dots$ find a sufficient and necessary
condition that the fraction $ \frac{1}{2n\plus{}1}$ has the same properties as above and find two such fractions other than $ \frac{1}{7}$.
1. **Understanding the Periodicity Condition:**
   For a fraction $\frac{1}{m}$ to have a repeating decimal period $T$, it must satisfy the condition that $m$ divides $10^T - 1$. This is because the decimal expansion of $\frac{1}{m}$ repeats every $T$ digits if and only if $10^T \equiv 1 \pmod{m}$.

2. **Minimal Periodicity:**
   The period $T$ is the smallest positive integer such that $10^T \equiv 1 \pmod{m}$. This is known as the order of 10 modulo $m$.

3. **Condition for Even Period:**
   If $T = 2n$ is even, then $10^T - 1 = (10^n - 1)(10^n + 1)$. For $m$ to divide $10^T - 1$, it must divide either $10^n - 1$ or $10^n + 1$. However, for the period to be exactly $2n$, $m$ must divide $10^n + 1$ but not $10^n - 1$.

4. **Sum of Periodic Parts:**
   For the fraction $\frac{1}{m}$ to have the property that the sum of the digits in one period equals $999$, the period $T$ must be 6, and the sum of the digits in the repeating sequence must be $999$. This implies that the repeating sequence is of the form $abc + def = 999$.

5. **Finding Suitable $m$:**
   We need to find $m = 2n + 1$ such that $m$ divides $10^6 - 1$ and the period is 6. We know $10^6 - 1 = 999999 = 3^3 \cdot 7 \cdot 11 \cdot 13 \cdot 37$. Therefore, $m$ must be a divisor of $999999$ that is of the form $2n + 1$ and results in a period of 6.

6. **Checking Divisors:**
   The divisors of $999999$ that are of the form $2n + 1$ include:
   - $7$ (already given)
   - $37$
   - $3^3 = 27$ (not of the form $2n + 1$)
   - $11$
   - $13$

   We need to check which of these have a period of 6:
   - For $m = 37$, $\frac{1}{37} = 0.\overline{027}$, which has a period of 3, not 6.
   - For $m = 11$, $\frac{1}{11} = 0.\overline{09}$, which has a period of 2, not 6.
   - For $m = 13$, $\frac{1}{13} = 0.\overline{076923}$, which has a period of 6.

Therefore, the fractions $\frac{1}{7}$ and $\frac{1}{13}$ have the desired properties.

The final answer is $\boxed{\frac{1}{13}}$ and $\boxed{\frac{1}{7}}$.