## Differential and Integral Equations

- Differential Integral Equations
- Volume 8, Number 4 (1995), 717-728.

### On the characteristic exponent function for a first order ordinary differential equation

Kurt Munk Andersen and Allan Sandqvist

#### Abstract

The paper deals with an equation $\dot x=f(t,x)$, $(t,x)\in I\times \Bbb R$, where $[0,1]\subset I$. The notion characteristic exponent for a {\it closed} solution is extended to an {\it arbitrary} solution $\varphi(t)$ defined on $[0,1]$---in the obvious way. If such solutions exist, the characteristic exponent as a function $\mu=\mu(\xi)$, $\xi=\varphi(0)$ is defined. This function is investigated by means of two formulae. The first one is a formula for the sum $\mu(\xi_1)+\mu(\xi_2)$, the second one is a formula for the value $\mu(\xi_2)$ in terms of arbitrary values $\xi_1$ and $\xi_3$ such that $\xi_1<\xi_2<\xi_3$. Besides these formulae a main result is that $\mu(\xi)$ is weakly convex if $f_x'(t,x)$ is weakly convex in $x$ for all fixed $t$. Moreover, the Riccati equation (in several ways) is characterized inside the class of equations where $f'_x(t,x)$ has the mentioned convexity property. Finally, an earlier result on closed solutions is completed by a stability discussion.

#### Article information

**Source**

Differential Integral Equations, Volume 8, Number 4 (1995), 717-728.

**Dates**

First available in Project Euclid: 20 May 2013

**Permanent link to this document**

https://projecteuclid.org/euclid.die/1369055607

**Mathematical Reviews number (MathSciNet)**

MR1306588

**Zentralblatt MATH identifier**

0815.34040

**Subjects**

Primary: 34C99: None of the above, but in this section

Secondary: 34D99: None of the above, but in this section

#### Citation

Andersen, Kurt Munk; Sandqvist, Allan. On the characteristic exponent function for a first order ordinary differential equation. Differential Integral Equations 8 (1995), no. 4, 717--728. https://projecteuclid.org/euclid.die/1369055607