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 The Solution of the Riccati Equation

In any mathematical investigation of the dynamics of a system, the introduction of a nonlinearity leads to some form of the Riccati equation:

#### dy/dt = a(t) + b(t)y + c(t)y2

Because the Riccati equation does not lend itself to solution by the techniques that produce solutions to linear differential equations typically the analysis is stalled. Moreover because of the similarity of the Riccati equation to the equations analyzed in Chaos Theory there is a presumption that further analysis will involve the concepts of Chaos Theory. But there is a line of analysis of the Riccati equation that is relatively simple.

The first important result in the analysis of the Riccati equation is that if one solution is known then a whole family of solutions can be found.

Let x(t) be a solution to the Riccati equation and suppose z(t) is a function such that

#### y(t) = x(t) + 1/z(t)

is a solution to the Riccati equation. As is shown in Appendix I this requires that z(t) satisfy the linear differential equation

#### dz/dt = -(b(t) + 2c(t)x(t))z - c(t)

This is an equation of the form

#### dz/dt = A(t)z - B(t)

which has a solution of

#### z(t) = eA(t)[∫te-A(s)B(s)ds + λ]

where λ is an arbitrary constant. Thus the solutions for z(t) can be determined as a one-parameter family of functions and hence the solutions to the Riccati equation can be expressed as a one-parameter family of functions.

But it is usually the case that not even one solution of the Riccati equation is known.

The second important result in the analysis of the solution of the Riccati equation is that the general equation can be reduced to a simpler form by the substitution

#### w(t) = exp(∫tc(s)ds)y(t) which leads to the equation dw/dt = p(t) + q(t)w2

where p(t)=-a(t)exp(∫tc(s)ds) and q(t)=-b(t)exp(-∫tc(s)ds).

A substitution of -Q(t)w(t)=(dz/dt)/z reduces the previous equation to

#### d/dt[-(1/q(t))du/dt] - p(t)u(t) = 0

This second order differential equation may lend itself to solution more readily than the previous forms of the Riccati equation.