A rational difference equation is a nonlinear difference equation of the form[1][2][3][4]

where the initial conditions are such that the denominator never vanishes for any n.

First-order rational difference equation

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

When and the initial condition are real numbers, this difference equation is called a Riccati difference equation.[3]

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

Equations of this form arise from the infinite resistor ladder problem.[5][6]

Solving a first-order equation

First approach

One approach[7] to developing the transformed variable , when , is to write

where and and where .

Further writing can be shown to yield

Second approach

This approach[8] gives a first-order difference equation for instead of a second-order one, for the case in which is non-negative. Write implying , where is given by and where . Then it can be shown that evolves according to

Third approach

The equation

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

where all of A, B, C, E, and X are n×n matrices (in this case n = 1); the solution of this is[9]

where

Application

It was shown in [10] that a dynamic matrix Riccati equation of the form

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.

References

  1. Skellam, J.G. (1951). “Random dispersal in theoretical populations”, Biometrika 38 196−–218, eqns (41,42)
  2. Camouzis, Elias; Ladas, G. (November 16, 2007). Dynamics of Third-Order Rational Difference Equations with Open Problems and Conjectures. CRC Press. ISBN 9781584887669 via Google Books.
  3. 1 2 Kulenovic, Mustafa R. S.; Ladas, G. (July 30, 2001). Dynamics of Second Order Rational Difference Equations: With Open Problems and Conjectures. CRC Press. ISBN 9781420035384 via Google Books.
  4. Newth, Gerald, "World order from chaotic beginnings", Mathematical Gazette 88, March 2004, 39-45 gives a trigonometric approach.
  5. "Equivalent resistance in ladder circuit". Stack Exchange. Retrieved 21 February 2022.
  6. "Thinking Recursively: How to Crack the Infinite Resistor Ladder Puzzle!". Youtube. Retrieved 21 February 2022.
  7. Brand, Louis, "A sequence defined by a difference equation," American Mathematical Monthly 62, September 1955, 489492. online
  8. Mitchell, Douglas W., "An analytic Riccati solution for two-target discrete-time control," Journal of Economic Dynamics and Control 24, 2000, 615622.
  9. 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.
  10. Balvers, Ronald J., and Mitchell, Douglas W., "Reducing the dimensionality of linear quadratic control problems," Journal of Economic Dynamics and Control 31, 2007, 141159.

Further reading

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