# Repertoire Method Clarification Required ( Concrete Mathematics )

In the book Concrete Mathematics, chapter 2, section 2.2 -- sums and recurrences, page 26 (2nd edition), the authors talk about the following example:

Given the general recurrence

$$R(0) = \alpha$$ $$R(n) = R(n-1) + \beta + \epsilon n$$

The authors generalize the recurrence relation to:

$$R(n) = A(n)\alpha + B(n)\beta + C(n)\epsilon$$

Employing the Repertoire Method, the authors plug in simple functions of $n$ in order to determine $A(n), B(n), C(n)$. So they discover:

Setting $R(n) = 1$ implies $\alpha = 1, \beta = 0, \epsilon = 0 \implies A(n) = 1$.

Setting $R(n) = n$ implies $\alpha = 0, \beta = 1, \epsilon = 0 \implies B(n) = n$.

Setting $R(n) = n^2$ implies $\alpha = 0, \beta = -1, \epsilon = 0 \implies C(n) = \frac{n^2 + n}{2}$.

Values for the first couple of terms of the recurrence:

$$\begin{eqnarray*} R(0) &=& \alpha \\ R(1) &=& \alpha + \beta + \epsilon \\ R(2) &=& \alpha + 2\beta + 3\epsilon \\ R(3) &=& \alpha + 3\beta + 6\epsilon \\ R(4) &=& \alpha + 4\beta + 10\epsilon \\ R(5) &=& \alpha + 5\beta + 15\epsilon \end{eqnarray*}$$

I do not understand what is the process through which the values for $\alpha$, $\beta$, and $\epsilon$ are implied. I would like some help with that. Where exactly do we look and what do we math them against to see what they have to be?

• (Off-topic remark: It's a gamma ($\gamma$) and not an epsilon ($\epsilon$). Sep 27, 2011 at 15:00
• My apologies for having disturbed the balance of left and right parentheses in the universe for more than 9 years with my previous comment!) Dec 18, 2020 at 10:02

Put $R_n=1$ (for all $n$; hence also $R_0$ and $R_{n-1}$ should be set equal to 1) in (2.7): $$1 = \alpha, \quad 1 = 1 + \beta + \gamma n.$$ The first equation tells us $\alpha$ right away, and the second equality holds for all $n$ iff $\beta=\gamma=0$.

Then put $R_n=n$ (hence $R_0=0$ and $R_{n-1}=n-1$) in (2.7): $$0 = \alpha, \quad n = (n-1)+\beta + \gamma n.$$ Here $\beta=1$ and $\gamma=0$ is required for the identity to hold for all $n$ (compare coefficients for the constant terms and for the $n$-terms separately).

Etc.

• What I don't get is that when choosing higher order functions for Rn, ie. Rn = n^2, I don't see how alpha, beta, and gamma can be guessed since there's no n^2 term in the right hand side of the equation. Sep 28, 2011 at 8:23
• With $R_n=n^2$ you get $0^2=\alpha$ and $n^2=(n-1)^2 + \beta + \gamma n$. In other words: $\alpha=0$ and $n^2 = n^2 + (\gamma-2)n + (1 + \beta)$, so that $\beta=-1$ and $\gamma=2$. Sep 28, 2011 at 8:29
• (Where I expanded $(n-1)^2=n^2-2n+1$ of course.) Sep 28, 2011 at 8:31
• Nevermind, I got it :). Oh, thanks anyway. :) Sep 28, 2011 at 8:31
• how can you be sure that the solution is of the form A(n)alpha.....
– user39338
Sep 3, 2012 at 13:28