A recurrence tells you how to get the next term, but computing $a_{100}$ step by step is slow. A closed form gives $a_{n}$ directly. For many first-order recurrences you can find one by substituting the rule into itself and recognizing a sum, often a geometric series.
Idea clave
To unroll $a_{n} = c\,a_{n-1} + d$: replace $a_{n-1}$ by $c\,a_{n-2} + d$, then $a_{n-2}$ by $c\,a_{n-3} + d$, and so on, until the pattern is clear and you reach the starting value. Then check the closed form by substituting it into the recurrence and the starting value (an induction proof).
Por qué funciona
Each substitution pushes the index down by one and adds one more term to a growing sum. After $j$ steps, $a_{n} = c^{j}a_{n-j} + d(c^{j-1} + \cdots + c + 1)$. Going all the way to the start turns the sum into a geometric series with a known total. The check matters because "the pattern is clear" is a guess until it is verified; unrolling suggests the answer, and induction confirms it.
Worked examples
Ejemplo resuelto: The Tower of Hanoi, solved
Solve $h_{n} = 2h_{n-1} + 1$ with $h_{1} = 1$.
- Unroll: $$h_{n} = 2(2h_{n-2} + 1) + 1 = 4h_{n-2} + 2 + 1 = 8h_{n-3} + 4 + 2 + 1 = \cdots$$
- After reaching $h_{1}$: $$h_{n} = 2^{n-1}h_{1} + 2^{n-2} + \cdots + 2 + 1 = 2^{n-1} + 2^{n-2} + \cdots + 1.$$
- The geometric sum is $2^{n} - 1$.
- Check: $h_{1} = 2 - 1 = 1$, and $2(2^{n-1} - 1) + 1 = 2^{n} - 1$. It satisfies both.
Respuesta: $h_{n} = 2^{n} - 1$
Ejemplo resuelto: Adding $n$ each time
Solve $a_{n} = a_{n-1} + n$ with $a_{0} = 0$.
- Unroll: $a_{n} = a_{n-2} + (n - 1) + n = \cdots = a_{0} + 1 + 2 + \cdots + n$.
- So $a_{n} = \frac{n(n + 1)}{2}$.
- Check: $\frac{(n-1)n}{2} + n = \frac{n^{2} - n + 2n}{2} = \frac{n(n + 1)}{2}$.
Respuesta: $a_{n} = \frac{n(n + 1)}{2}$
Ejemplo resuelto: A recurrence with subtraction
Solve $a_{n} = 3a_{n-1} - 4$ with $a_{0} = 5$.
- Unroll: $$a_{n} = 3^{n}a_{0} - 4(3^{n-1} + \cdots + 3 + 1) = 5 \cdot 3^{n} - 4 \cdot \frac{3^{n} - 1}{2}.$$
- Simplify: $5 \cdot 3^{n} - 2 \cdot 3^{n} + 2 = 3 \cdot 3^{n} + 2 = 3^{n+1} + 2$.
- Check: $a_{0} = 3 + 2 = 5$, and $3(3^{n} + 2) - 4 = 3^{n+1} + 2$.
Respuesta: $a_{n} = 3^{n+1} + 2$
A shortcut for $a_{n} = c\,a_{n-1} + d$ with $c \ne 1$: find the fixed point $p$ with $p = cp + d$, so $p = \frac{d}{1 - c}$. Subtracting $p = cp + d$ from the recurrence gives $a_{n} - p = c(a_{n-1} - p)$: the distance from $p$ is multiplied by $c$ each step. So $a_{n} = p + c^{n}(a_{0} - p)$. In the last example $p = 2$ and $a_{0} - p = 3$, giving $2 + 3 \cdot 3^{n}$ again.
Error común
Guessing a formula from the first two terms and never checking it. Hanoi starts $1, 3$, which fits $2n - 1$; but the next term is $7$, not $5$. Two terms fit a straight line through them, whatever the sequence really is. Check: compute a third term (or more) from the recurrence, and substitute your formula into the recurrence itself.