If $ a,n $ are two natural numbers corelated by the equation $ \left\{ \sqrt{n+\sqrt n}\right\} =\left\{ \sqrt a\right\} , $ then $ 
1+4a $ is a perfect square. Justify this statement. Here, $ \{\} $ is the usual fractionary part.
1. **Lemma Application**:
   We use the lemma: If \(z\sqrt{x} - t\sqrt{y} = u\) for \(x, y, z, t, u \in \mathbb{Z}\) and \(x, y \geq 0\), then either some of \(z, t, u\) is 0 or \(x\) and \(y\) are both perfect squares.

2. **Define \(d\)**:
   Let \(d = \sqrt{n + \sqrt{n}} - \sqrt{a}\). We have:
   \[
   d = \sqrt{n + \sqrt{n}} - \sqrt{a} = \left(\left\lfloor \sqrt{n + \sqrt{n}} \right\rfloor + \left\{ \sqrt{n + \sqrt{n}} \right\}\right) - \left(\left\lfloor \sqrt{a} \right\rfloor + \left\{ \sqrt{a} \right\}\right)
   \]
   Since the fractional parts are equal, the integer parts must differ by an integer:
   \[
   d = \left\lfloor \sqrt{n + \sqrt{n}} \right\rfloor - \left\lfloor \sqrt{a} \right\rfloor \in \mathbb{Z}
   \]

3. **Square Both Sides**:
   \[
   d + \sqrt{a} = \sqrt{n + \sqrt{n}}
   \]
   Squaring both sides:
   \[
   (d + \sqrt{a})^2 = n + \sqrt{n}
   \]
   Expanding and simplifying:
   \[
   d^2 + 2d\sqrt{a} + a = n + \sqrt{n}
   \]
   Isolate the term involving \(\sqrt{n}\):
   \[
   \sqrt{n} - 2d\sqrt{a} = d^2 + a - n
   \]

4. **Apply the Lemma**:
   According to the lemma, we have three cases to consider:
   - \(d = 0\)
   - \(d^2 + a - n = 0\)
   - \(n\) and \(a\) are both perfect squares

5. **Case 1: \(d = 0\)**:
   If \(d = 0\), then:
   \[
   \sqrt{n + \sqrt{n}} = \sqrt{a}
   \]
   This implies:
   \[
   n + \sqrt{n} = a
   \]
   Since \(n\) is a natural number, \(n\) must be a perfect square. Let \(n = t^2\) for some \(t \in \mathbb{N}\). Then:
   \[
   a = t^2 + t
   \]
   Now, we check if \(1 + 4a\) is a perfect square:
   \[
   1 + 4a = 1 + 4(t^2 + t) = 1 + 4t^2 + 4t = (2t + 1)^2
   \]
   Thus, \(1 + 4a\) is indeed a perfect square.

6. **Case 2: \(d^2 + a - n = 0\)**:
   If \(d^2 + a - n = 0\), then:
   \[
   \sqrt{n} - 2d\sqrt{a} = 0
   \]
   This implies:
   \[
   \sqrt{n} = 2d\sqrt{a}
   \]
   Squaring both sides:
   \[
   n = 4d^2a
   \]
   Substituting \(n = d^2 + a\) into the equation:
   \[
   d^2 + a = 4d^2a
   \]
   Rearranging:
   \[
   4d^2a - d^2 = a
   \]
   \[
   d^2(4a - 1) = a
   \]
   Since \(a\) and \(d\) are natural numbers, this equation is not generally solvable for natural numbers \(a\) and \(d\).

7. **Case 3: \(n\) and \(a\) are both perfect squares**:
   Let \(n = t^2\) and \(a = u^2\) for some \(t, u \in \mathbb{N}\). Then:
   \[
   \sqrt{t^2 + t} = d + u
   \]
   This implies:
   \[
   t^2 < t^2 + t < (t + 1)^2
   \]
   Hence, \(d + u\) cannot be an integer, making this case impossible.

Thus, the only valid case is when \(d = 0\), leading to \(1 + 4a\) being a perfect square.

The final answer is \( \boxed{ 1 + 4a = (2t + 1)^2 } \)