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Rational difference equation

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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.

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First-order rational difference equation

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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]

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Solving a first-order equation

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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

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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

Further reading

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