Rational difference equation

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A rational difference equation is a nonlinear difference equation of the form[1][2][3]

x_{n+1} = \frac{\alpha+\sum_{i=0}^k \beta_ix_{n-i}}{A+\sum_{i=0}^k B_ix_{n-i}}~,

where the initial conditions x_{0}, x_{-1},\dots, x_{-k} are such that the denominator never vanishes for any n.

First-order rational difference equation[edit]

A first-order rational difference equation is a nonlinear difference equation of the form

w_{t+1} = \frac{aw_t+b}{cw_t+d}.

When a,b,c,d and the initial condition w_{0} are real numbers, this difference equation is called a Riccati difference equation.[3]

Such an equation can be solved by writing w_t as a nonlinear transformation of another variable x_t which itself evolves linearly. Then standard methods can be used to solve the linear difference equation in x_t.

Solving a first-order equation[edit]

First approach[edit]

One approach [4] to developing the transformed variable x_t, when ad-bc \neq 0, is to write

y_{t+1}= \alpha - \frac{\beta}{y_t}

where \alpha = (a+d)/c and \beta = (ad-bc)/c^{2} and where w_t = y_t -d/c.

Further writing y_t = x_{t+1}/x_t can be shown to yield

x_{t+2} - \alpha x_{t+1} + \beta x_t =0. \,

Second approach[edit]

This approach [5] gives a first-order difference equation for x_t instead of a second-order one, for the case in which (d-a)^{2}+4bc is non-negative. Write x_t = 1/(\eta + w_t) implying w_t = (1- \eta x_t)/x_t, where \eta is given by \eta = (d-a+r)/2c and where r=\sqrt{(d-a)^{2}+4bc}. Then it can be shown that x_t evolves according to

x_{t+1} =\left( \frac{d-\eta c}{\eta c+a}\right)x_t + \frac{c}{\eta c+a}.

Third approach[edit]

The equation

w_{t+1} = \frac{aw_t+b}{cw_t+d}

can also be solved by treating it as a special case of the more general matrix equation

X_{t+1} = -(E+BX_t)(C+AX_t)^{-1},

where all of A, B, C, E, and X are n×n matrices (in this case n=1); the solution of this is[6]



\begin{pmatrix} N_{t} \\ D_{t}\end{pmatrix} = \begin{pmatrix} -B & -E \\ A & C \end{pmatrix}^t\begin{pmatrix} X_0\\ I \end{pmatrix}.


It was shown in [7] that a dynamic matrix Riccati equation of the form

 H_{t-1} = K +A'H_tA - A'H_tC(C'H_tC)^{-1}C'H_tA, \,

which can arise in some discrete-time optimal control problems, can be solved using the second approach above if the matrix C has only one more row than column.


  1. ^ Skellam, J.G. (1951). “Random dispersal in theoretical populations”, Biometrika 38 196−–218, eqns (41,42)
  2. ^ Dynamics of third-order rational difference equations with open problems and Conjectures
  3. ^ a b Dynamics of Second-order rational difference equations with open problems and Conjectures
  4. ^ Brand, Louis, "A sequence defined by a difference equation," American Mathematical Monthly 62, September 1955, 489–492. online
  5. ^ Mitchell, Douglas W., "An analytic Riccati solution for two-target discrete-time control," Journal of Economic Dynamics and Control 24, 2000, 615–622.
  6. ^ Martin, C. F., and Ammar, G., "The geometry of the matrix Riccati equation and associated eigenvalue method," in Bittani, Laub, and Willems (eds.), The Riccati Equation, Springer-Verlag, 1991.
  7. ^ Balvers, Ronald J., and Mitchell, Douglas W., "Reducing the dimensionality of linear quadratic control problems," Journal of Economic Dynamics and Control 31, 2007, 141–159.

See also[edit]

  • Simons, Stuart, "A non-linear difference equation," Mathematical Gazette 93, November 2009, 500-504.