Find all rational numbers $a$,for which there exist infinitely many positive rational numbers $q$ such that the equation 
$[x^a].{x^a}=q$ has no solution in rational numbers.(A.Vasiliev)
1. **Restate the problem in terms of function \( f_a(x) \):**
   We need to find all rational numbers \( a \) such that the function \( f_a(x) = \lfloor x^a \rfloor \{ x^a \} \) does not cover infinitely many points of \( \mathbb{Q}^+ \).

2. **Simplify the problem:**
   Since \( \{ x^a \} \in \mathbb{Q} \), it implies \( x^a \in \mathbb{Q} \). If \( x = \left( \frac{m}{k} \right)^{\frac{1}{a}} \) where \( \gcd(m, k) = 1 \), then \( x^a = \frac{m}{k} \) is rational. This means we can reduce the problem to checking whether \( \frac{m}{k} \) is a solution.

3. **Consider the function for negative exponents:**
   Note that \( f_a(x) = f_{-a} \left( \frac{1}{x} \right) \). Therefore, it is sufficient to solve the problem for \( a \in \mathbb{N}_0 \).

4. **Check specific values of \( a \):**
   - For \( a = 0 \), \( f_0(x) = \lfloor x^0 \rfloor \{ x^0 \} = \lfloor 1 \rfloor \cdot \{ 1 \} = 0 \). This function does not cover any positive rational numbers, so \( a = 0 \) is a solution.
   - For \( a = 1 \), \( f_1(x) = \lfloor x \rfloor \{ x \} \). By choosing \( x = n + \frac{1}{m} \) for integers \( n \) and \( m \), we can generate any rational number \( \frac{n}{m} \). Thus, \( a = 1 \) does not work.

5. **Generalize for \( a = \frac{1}{n} \):**
   - For \( a = \frac{1}{n} \), we need to show that \( f_{\frac{1}{n}}(x) \) does not cover infinitely many points of \( \mathbb{Q}^+ \). This is equivalent to showing that \( f_n(x) = \frac{1}{t^n} \) has no solution for natural \( t \).

6. **Prove for \( a = n \) where \( n \in \mathbb{N}_{>1} \):**
   - Let \( x = \frac{a}{b} \) and \( a^n = b^n \cdot c + r \) where \( \gcd(a, b) = \gcd(b, r) = 1 \) and \( r < b^n \). We need to solve \( f_n(x) = \frac{cr}{b^n} = \frac{1}{t^n} \).
   - Since \( r \mid b^n \) and \( \gcd(r, b) = 1 \), we get \( r = 1 \). Thus, \( t^n \mid b^n \) implies \( b = ts \) for some natural \( s \), and \( c = s^n \).
   - This gives \( a^n = (bs)^n + 1 \). By Fermat's Last Theorem, this has no solution for \( n \geq 3 \). For \( n = 2 \), we get \( (a - bs)(a + bs) = 1 \), which has no solution in positive integers.

7. **Conclusion:**
   - All \( a \in \mathbb{N}_{>1} \) and \( a = 0 \) are solutions.
   - Therefore, all \( a = \frac{n}{z} \) where \( n \in \mathbb{N}_{>1} \cup \{0\} \) and \( z \in \mathbb{Z}_{\ne 0} \) are solutions.

The final answer is \( \boxed{ \mathbb{Q} \setminus \left\{ \frac{1}{n} \mid n \in \mathbb{Z}_{\ne 0} \right\} } \).