The Wronskian is defined by
for given functions and .
Let be the Wronskian of two solutions and of the differential equations
where and are constants.
- Prove that satisfies the first-order linear differential equation
By this formula we can see that if then for all .
- Assume is not identically zero and prove that if and only if is constant.
- First, we evaluate where is the Wronskian of the two functions and .
Furthermore, by Theorem 8.3 (page 310 of Apostol), since is a solution of we know
since . Hence, if .
- Assume . Then for all . By part (a) of the previous exercise (Section 8.14, Exercise #21) we know is constant.
Conversely, assume is constant. Then, again by the previous exericse, we have for all . Hence,