To determine at which differences the polynomial sequence \( a_n = 2n^4 - n^3 \) reaches a constant value, we should look at the degree of the polynomial.
The given polynomial \( a_n \) is of degree 4, which means its highest power of \( n \) is \( n^4 \).
For any polynomial of degree \( k \), the \( (k+1) \)-th differences will be constant. Therefore, since our polynomial is of degree 4, we need to consider the 5th differences:
- 1st differences are not constant.
- 2nd differences are not constant.
- 3rd differences are not constant.
- 4th differences are not constant.
- 5th differences will be constant.
Thus, the polynomial sequence \( a_n = 2n^4 - n^3 \) reaches a constant value at the 5th differences.