In arithmetic recursive sequences or equations, the use of terms like $f(n-1)$, $f(n-2)$, etc., instead of just $f(n-1)$, depends on the complexity of the relationship between terms in the sequence.

Here's why different terms might be used:

1. Complex Dependencies:

   • If the sequence is defined by a relationship that involves more than one previous term, you'll need more than just $f(n-1)$. For example, the Fibonacci sequence is defined as $f(n) = f(n-1) + f(n-2)$, because each term depends on the sum of the two preceding terms.

2. Initial Conditions:

   • Multiple terms like $f(n-2)$ or $f(n-3)$ might be needed to set initial conditions correctly. The number of initial conditions corresponds to the number of terms in the recursive equation. For example, if you need two initial conditions, you likely have an equation involving $f(n-1)$ and $f(n-2)$.

3. Higher-Order Recurrence:

   • In sequences where the relationship is more complex, higher-order recurrence relations (which use more than one previous term) provide a more accurate or necessary description of the sequence. These might describe phenomena where the future state depends on multiple past states.

4. Modeling Specific Problems:

   • Some problems naturally require references to multiple previous terms. For instance, in certain financial models or biological processes, the current state might depend on both the immediate past state and the state before that.

Why not just use $f(n-1)$?

If you were to use only $f(n-1)$, it implies that each term in the sequence only depends directly on the immediately preceding term. While this is simpler, it cannot model situations where the current term is influenced by multiple prior terms.

Conclusion:

The choice between $f(n-1)$, $f(n-2)$, or even higher terms depends on how the sequence is defined and what real-world phenomenon it is modeling. Simpler sequences use only $f(n-1)$, but more complex sequences may require additional terms.

Would you like further details or examples?

Here are some related questions:

1. What is an example of a sequence that uses only $f(n-1)$?
2. Can you explain the Fibonacci sequence in more detail?
3. How do initial conditions affect recursive sequences?
4. What is a second-order recurrence relation?
5. Are there sequences that use more than two previous terms?
6. What role do recurrence relations play in algorithm design?
7. Can you solve a recurrence relation involving $f(n-1)$ and $f(n-2)$?
8. What is the general solution to a linear recurrence relation?

Tip: When analyzing recursive sequences, always consider the number of initial conditions required, as they indicate the order and complexity of the recurrence relation.