Let $ I $ and $ 0 $ be the square identity and null matrices, both of size $ 2019 $. There is a square matrix $A$
with rational entries and size $ 2019 $ such that:
a) $ A ^ 3 + 6A ^ 2-2I = 0 $?
b) $ A ^ 4 + 6A ^ 3-2I = 0 $?
To determine whether there exists a square matrix \( A \) of size \( 2019 \) with rational entries that satisfies the given equations, we need to analyze the characteristic polynomial and minimal polynomial of \( A \).

### Part (a): \( A^3 + 6A^2 - 2I = 0 \)

1. **Characteristic Polynomial and Minimal Polynomial:**
   - The given equation can be interpreted as a polynomial equation in \( A \):
     \[
     A^3 + 6A^2 - 2I = 0
     \]
   - This implies that the minimal polynomial \( m_A(x) \) of \( A \) divides the polynomial \( x^3 + 6x^2 - 2 \).

2. **Roots of the Polynomial:**
   - To understand the roots of the polynomial \( x^3 + 6x^2 - 2 \), we can use the Rational Root Theorem, which suggests that any rational root must be a factor of the constant term (-2) divided by the leading coefficient (1). The possible rational roots are \( \pm 1, \pm 2 \).
   - Testing these values:
     \[
     f(1) = 1^3 + 6 \cdot 1^2 - 2 = 1 + 6 - 2 = 5 \quad (\text{not a root})
     \]
     \[
     f(-1) = (-1)^3 + 6 \cdot (-1)^2 - 2 = -1 + 6 - 2 = 3 \quad (\text{not a root})
     \]
     \[
     f(2) = 2^3 + 6 \cdot 2^2 - 2 = 8 + 24 - 2 = 30 \quad (\text{not a root})
     \]
     \[
     f(-2) = (-2)^3 + 6 \cdot (-2)^2 - 2 = -8 + 24 - 2 = 14 \quad (\text{not a root})
     \]
   - Since none of the possible rational roots are actual roots, \( x^3 + 6x^2 - 2 \) has no rational roots and is irreducible over the rationals.

3. **Conclusion for Part (a):**
   - Since \( x^3 + 6x^2 - 2 \) is irreducible over the rationals, the minimal polynomial of \( A \) must be \( x^3 + 6x^2 - 2 \).
   - Therefore, there exists a matrix \( A \) with rational entries that satisfies \( A^3 + 6A^2 - 2I = 0 \).

### Part (b): \( A^4 + 6A^3 - 2I = 0 \)

1. **Characteristic Polynomial and Minimal Polynomial:**
   - The given equation can be interpreted as a polynomial equation in \( A \):
     \[
     A^4 + 6A^3 - 2I = 0
     \]
   - This implies that the minimal polynomial \( m_A(x) \) of \( A \) divides the polynomial \( x^4 + 6x^3 - 2 \).

2. **Irreducibility:**
   - To check if \( x^4 + 6x^3 - 2 \) is irreducible over the rationals, we can use Eisenstein's criterion with \( p = 2 \):
     - The polynomial \( x^4 + 6x^3 - 2 \) can be written as \( x^4 + 6x^3 + 0x^2 + 0x - 2 \).
     - Here, \( p = 2 \) divides the coefficients of \( x^3 \), \( x^2 \), and \( x \), but does not divide the leading coefficient (1) and the constant term (-2).
     - Additionally, \( p^2 = 4 \) does not divide the constant term (-2).
   - Therefore, by Eisenstein's criterion, \( x^4 + 6x^3 - 2 \) is irreducible over the rationals.

3. **Conclusion for Part (b):**
   - Since \( x^4 + 6x^3 - 2 \) is irreducible over the rationals, the minimal polynomial of \( A \) must be \( x^4 + 6x^3 - 2 \).
   - Therefore, there exists a matrix \( A \) with rational entries that satisfies \( A^4 + 6A^3 - 2I = 0 \).

The final answer is \( \boxed{\text{True}} \)