# Differential inclusion

In mathematics, differential inclusions are a generalization of the concept of ordinary differential equation of the form

$\frac{dx}{dt}(t)\in F(t,x(t)),$

where F is a multivalued map, i.e. F(t, x) is a set rather than a single point in $\scriptstyle{\Bbb R}^d$. Differential inclusions arise in many situations including differential variational inequalities, projected dynamical systems, dynamic Coulomb friction problems and fuzzy set arithmetic.

For example, the basic rule for Coulomb friction is that the friction force has magnitude μN in the direction opposite to the direction of slip, where N is the normal force and μ is a constant (the friction coefficient). However, if the slip is zero, the friction force can be any force in the correct plane with magnitude smaller than or equal to μN Thus, writing the friction force as a function of position and velocity leads to a set-valued function.

## Theory

Existence theory usually assumes that F(tx) is an upper hemicontinuous function of x, measurable in t, and that F(tx) is a closed, convex set for all t and x. Existence of solutions for the initial value problem

$\frac{dx}{dt}(t)\in F(t,x(t)), \quad x(t_0)=x_0$

for a sufficiently small time interval [t0t0 + ε), ε > 0 then follows. Global existence can be shown provided F does not allow "blow-up" ($\scriptstyle \Vert x(t)\Vert\,\to\,\infty$ as $\scriptstyle t\,\to\, t^*$ for a finite $\scriptstyle t^*$).

Existence theory for differential inclusions with non-convex F(tx) is an active area of research.

Uniqueness of solutions usually requires other conditions. For example, suppose $F(t,x)$ satisfies a one-sided Lipschitz condition:

$(x_1-x_2)^T(F(t,x_1)-F(t,x_2))\leq C\Vert x_1-x_2\Vert^2$

for some C for all x1 and x2. Then the initial value problem

$\frac{dx}{dt}(t)\in F(t,x(t)), \quad x(t_0)=x_0$

has a unique solution.

This is closely related to the theory of maximal monotone operators, as developed by Minty and Haïm Brezis.

## Applications

Differential inclusions can be used to understand and suitably interpret discontinuous ordinary differential equations, such as arise for Coulomb friction in mechanical systems and ideal switches in power electronics. An important contribution has been made by A. F. Filippov, who studied regularizations of discontinuous equations. Further the technique of regularization was used by N.N. Krasovskii in the theory of differential games.

## References

• Aubin, Jean-Pierre; Cellina, Arrigo (1984). Differential Inclusions, Set-Valued Maps And Viability Theory. Grundl. der Math. Wiss. 264. Berlin: Springer. ISBN 9783540131052.
• Aubin, Jean-Pierre; Frankowska, Helene (1990). Set-Valued Analysis. Birkhäuser. ISBN 978-0817648473.
• Deimling, Klaus (1992). Multivalued Differential Equations. Walter de Gruyter. ISBN 978-3110132120.
• Andres, J.; Górniewicz, Lech (2003). Topological Fixed Point Principles for Boundary Value Problems. Springer. ISBN 978-9048163182.
• Filippov, A.F. (1988). Differential equations with discontinuous right-hand sides. Kluwer Academic Publishers Group. ISBN 90-277-2699-X.