To determine at which differences the polynomial sequence \( a_n = 2n^4 - n^3 \) reaches a constant value, we need to analyze the polynomial degree.
The polynomial \( a_n = 2n^4 - n^3 \) is a polynomial of degree 4. The general rule for polynomials is that the \( k \)-th finite differences of a polynomial of degree \( d \) become constant at the \( d \)-th difference.
In this case, since the polynomial is of degree 4, we'll look for the 4th differences to reach a constant value.
Thus, the answer is:
4th differences