## The Perron integral in ordinary differential equations.(English)Zbl 0784.34006

A finite sequence of numbers $$D=\{\alpha_ 0,\tau_ 1,\alpha_ 1,\cdots, \alpha_{k-1},\tau_ k,\alpha_ k\}$$ is called a partition of an interval $$[a,b]$$ if $$a=\alpha_ 0<\alpha_ 1< \cdots<\alpha_ k=b$$ and $$\alpha_{j-1}\leq \tau_ j\leq\alpha_ j$$, $$j=1,2,\dots,k$$. Given a positive function $$\delta: [a,b]\to (0,\infty)$$, called a gauge, the partition $$D$$ is called $$\delta$$-fine if $$[\alpha_{j-1},\alpha_ j]\subset [\tau_ j- \delta(\tau_ j), \tau_ j+\delta(\tau_ j)]$$, $$j=1,2,\dots,k$$. Let a function $$f:[a,b]\times \mathbb{R}^ n\to \mathbb{R}^ n$$ satisfy the following conditions: (1) $$f(t,\cdot)$$ is continuous for almost all $$t\in[a,b]$$, (2) the integral $$\int_ a^ b f(s,z)ds$$ in the sense of Perron exists for every $$z\in\mathbb{R}^ n$$, (3) there is a compact set $$S\subset\mathbb{R}^ n$$ and a gauge $$\delta$$ on $$[a,b]$$ such that for all $$\delta$$-fine partitions $$\{\alpha_ 0,\tau,\dots,\alpha_ k\}$$ and all functions $$w:[a,b]\to\mathbb{R}^ n$$ we have: $$\sum_{i=1}^ k f(\tau_ i, w(\tau_ i)) (\alpha_ i- \alpha_{i-1})\in S$$. The author shows that under these conditions for every $$v\in\mathbb{R}^ n$$ and $$\alpha\in[a,b]$$ there is a function $$y:[a,b]\to \mathbb{R}^ n$$ such that $$y(t)=v+ \int_ \alpha^ t f(s,y(s))ds$$ for $$t\in[a,b]$$, where the integral is understood in the sense of Perron.

### MSC:

 34A12 Initial value problems, existence, uniqueness, continuous dependence and continuation of solutions to ordinary differential equations 34A34 Nonlinear ordinary differential equations and systems 26A39 Denjoy and Perron integrals, other special integrals

### Keywords:

partition; gauge; integral; in the sense of Perron