I've got a sequence defined as such:
Between $0$ and $5$, $i = 1$
Between $6$ and $10$, $i = d$
Between $11$ and $15$, $i = d^2$
Between $16$ and $20$, $i = d^3$ ... I guess I could write it like this for $n=5$:
Between $k(n)+1$ and $(k+1)n$, $i= d^k$ and I'm looking for the $n$th term of the induced serie. (ie the sum of the $n$th first terms of my sequence)
Actually, I'm trying to solve a real-world problem here. We're selling products with a progressive discount (eg the first $5$ items are at full price, then the $5$ following items have a $10%$ discount, then there's an extra 10 percent discount for the next clip of $5$, etc).
So far I'm handling this kind of manually.