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# Linear difference equations

##### Definition
linear difference equation is a recurrence relation of the form $y_{t+n}=a_1y_{t+n-1}+a_2y_{t+n-2}+\cdots +a_ny_t+b$. If $b=0$, then it is called homogeneous. In this article, we shall also assume $t=0$ for convenience. The name “difference equation” is a nod to the fact that it is analogous to a differential equation if we take finite differences instead of taking limits. (Note that, $\frac{dy}{dx}=\lim_{h\to 0}\frac{y(x+h)-y(x)}{h}=\lim_{\Delta x\to 0}\frac{\Delta y}{\Delta x}$.) In what follows, we shall illustrate a common method to explicitly solve such equations.

##### The solution space
Note that, if two sequences $z_n$ and $w_n$ are solutions to the given equation (in the homogeneous case), then $az_n+bw_n$ is also a solution for any two constants $a,b$. In such cases, the set of all solutions is said to possess a linear structure. It means that the solutions can be thought of as vectors in an abstract space (more on this later). If this space is simply $\mathbb{R}^2$ or $\mathbb{R}^3$, then we can use our knowledge of vectors in these spaces to find all solutions in terms of a few special ones.

##### A useful example
Consider the equation $y_n=py_{n-1}+qy_{n-2}$. Note that, given $y_0$ and $y_1$, $y_n$ is determined for all $n\ge 2$. Thus, there exists a one-to-one correspondence between the set of all solutions (let us call it $\Gamma$) and the set of all $(y_0,y_1)$ (the latter is just $\mathbb{R}^2$). Hence, $\Gamma$ is nothing but $\mathbb{R}^2$ (in linear algebra, this correspondence is called a linear isomorphism). Note that, given two non-colinear vectors in the plane, any third vector can be written as a linear combination of the first two (please check!). Thus, given the solutions corresponding to two such vectors, any other solution can be written as a linear combination of them.
##### The method
Let us look for solutions of the form $y_n=\lambda^n$. Plugging into the equation, we get $\lambda^2-p\lambda-q=0$ (this is called the characteristic equation of the recurrence). Solving, we get two values $\lambda_+$ and $\lambda_-$. If they are different, then the corresponding vectors in $\mathbb{R}^2$ are not colinear (check!). Hence, given any solution $x_n$ there exist $a,b$ such that $x_n=a\lambda_+^n+b\lambda_-^n$. If the roots are the same, then the $x_n$ maybe written in the form $(a+bn)\lambda^n$ (why?)

The above method generalises to difference equations with solution spaces of dimension higher than 2 if the roots of the characteristic equation are all distinct. However, the trick used to deal with coincident roots in the example does not generalise immediately to higher dimensions.

The non-homogeneous case can be transformed into the homogeneous case by making a linear change of variables. See more on this here.

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