We consider the real numbers $a _ 1, a _ 2, a _ 3, a _ 4, a _ 5$ with the zero sum and the property that $| a _ i - a _ j | \le 1$ , whatever it may be $i,j \in \{1, 2, 3, 4, 5 \} $. Show that $a _ 1 ^ 2 + a _ 2 ^ 2 + a _ 3 ^ 2 + a _ 4 ^ 2 + a _ 5 ^ 2 \le \frac {6} {5}$ .
1. Given the real numbers \(a_1, a_2, a_3, a_4, a_5\) with the properties:
   - \(a_1 + a_2 + a_3 + a_4 + a_5 = 0\)
   - \(|a_i - a_j| \leq 1\) for all \(i, j \in \{1, 2, 3, 4, 5\}\)

2. We start by considering the square of the sum of these numbers:
   \[
   (a_1 + a_2 + a_3 + a_4 + a_5)^2 = 0
   \]
   Expanding this, we get:
   \[
   a_1^2 + a_2^2 + a_3^2 + a_4^2 + a_5^2 + 2 \sum_{i < j} a_i a_j = 0
   \]
   Therefore:
   \[
   a_1^2 + a_2^2 + a_3^2 + a_4^2 + a_5^2 = -2 \sum_{i < j} a_i a_j
   \]

3. Let \(S = \sum_{i < j} a_i a_j\). Then:
   \[
   \sum_{i=1}^5 a_i^2 = -2S
   \]

4. Next, we use the condition \(|a_i - a_j| \leq 1\). Consider the sum of the squares of the differences:
   \[
   \sum_{i < j} (a_i - a_j)^2
   \]
   Expanding this, we get:
   \[
   \sum_{i < j} (a_i - a_j)^2 = \sum_{i < j} (a_i^2 - 2a_i a_j + a_j^2)
   \]
   This can be rewritten as:
   \[
   \sum_{i < j} (a_i - a_j)^2 = \sum_{i < j} a_i^2 + \sum_{i < j} a_j^2 - 2 \sum_{i < j} a_i a_j
   \]
   Since each \(a_i^2\) appears in the sum \(\sum_{i < j} a_i^2\) exactly 4 times (because there are 4 other \(a_j\) for each \(a_i\)):
   \[
   \sum_{i < j} a_i^2 + \sum_{i < j} a_j^2 = 4 \sum_{i=1}^5 a_i^2
   \]
   Therefore:
   \[
   \sum_{i < j} (a_i - a_j)^2 = 4 \sum_{i=1}^5 a_i^2 - 2 \sum_{i < j} a_i a_j
   \]
   Using \(\sum_{i=1}^5 a_i^2 = -2S\), we get:
   \[
   \sum_{i < j} (a_i - a_j)^2 = 4(-2S) - 2S = -8S - 2S = -10S
   \]

5. Since \(|a_i - a_j| \leq 1\), we have:
   \[
   (a_i - a_j)^2 \leq 1
   \]
   There are \(\binom{5}{2} = 10\) such terms, so:
   \[
   \sum_{i < j} (a_i - a_j)^2 \leq 10
   \]
   Therefore:
   \[
   -10S \leq 10
   \]
   This implies:
   \[
   S \geq -1
   \]

6. Using \(\sum_{i=1}^5 a_i^2 = -2S\), and since \(S \geq -1\):
   \[
   \sum_{i=1}^5 a_i^2 \leq -2(-1) = 2
   \]

7. Finally, we need to show that \(\sum_{i=1}^5 a_i^2 \leq \frac{6}{5}\). From the inequality \(5 \sum_{i=1}^5 a_i^2 \leq 6\), we get:
   \[
   \sum_{i=1}^5 a_i^2 \leq \frac{6}{5}
   \]

\(\blacksquare\)

The final answer is \( \boxed{ \sum_{i=1}^5 a_i^2 \leq \frac{6}{5} } \)