Integral funnel
of a point
for a differential equation dx / dt = f ( t , x )
The set of all points lying on the integral curves (cf. Integral curve) passing through P . (By an equation one can mean a system of equations in vector notation with x = ( x _ {1} \dots x _ {n} ) .) If only one integral curve passes through P , then the integral funnel consists of this single curve. In the case n = 1 , that is, when x is scalar, the integral funnel consists of points ( t , x ) for which x _ {*} ( t ) \leq x \leq x ^ {*} ( t ) , where x ^ {*} ( t ) and x _ {*} ( t ) are the upper and lower solutions, that is, the largest and smallest solutions passing through P .
If the function f ( t , x ) is continuous (or satisfies the conditions of the Carathéodory existence theorem), then the integral funnel is a closed set. Furthermore, if all the solutions passing through P exist on the interval a \leq t \leq b , then this segment of the funnel (the part of the integral funnel defined by the inequalities a \leq t \leq b ) and the section of the integral funnel by any plane t = t _ {1} \in [ a , b ] are connected compact sets. Any point on the boundary of the integral funnel can be joined to P by a piece of the integral curve lying on the boundary of the integral funnel. If the sequence of points P _ {k} , k = 1 , 2 \dots converges to P , then the segments of the funnels of the points P _ {k} converge to the segment of the funnel of P in the sense that for any \epsilon > 0 they are contained in an \epsilon - neighbourhood of the segment of the funnel of P if k > k _ {1} ( \epsilon ) . Analogous properties are possessed by integral funnels for differential inclusions (cf. Differential inclusion)
\dot{x} \in F ( t , x )
under specified hypotheses concerning the set F ( t , x ) .
References
[1] | E. Kamke, "Zur Theorie der Systeme gewöhnlicher Differentialgleichungen. II" Acta Math. , 58 (1932) pp. 57–85 |
[2] | M.F. Bokstein, Uchen. Zap. Moskov. Gos. Univ. Ser. Mat. , 15 (1939) pp. 3–72 |
[3] | C.C. Pugh, "Funnel sections" J. Differential Eq. , 19 : 2 (1975) pp. 270–295 |
Comments
References
[a1] | A.F. Filippov, "Differential equations with discontinuous righthand sides" , Kluwer (1988) pp. 16 (Translated from Russian) |
Integral funnel. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Integral_funnel&oldid=47373