Solutions of Linear 2nd Order ODE have Common Zero iff Linearly Dependent
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Theorem
Let $\map {y_1} x$ and $\map {y_2} x$ be particular solutions to the homogeneous linear second order ODE:
- $(1): \quad \dfrac {\d^2 y} {\d x^2} + \map P x \dfrac {\d y} {\d x} + \map Q x y = 0$
on a closed interval $\closedint a b$.
Let $y_1$ and $y_2$ both have a zero for the same value of $x$ in $\closedint a b$.
Then $y_1$ and $y_2$ are constant multiples of each other.
That is, $y_1$ and $y_2$ are linearly dependent.
Proof
Let $\xi \in \closedint a b$ be such that $\map {y_1} \xi = \map {y_2} \xi = 0$.
Consider the Wronskian $\map W {y_1, y_2}$ at $\xi$:
\(\ds \map W {\map {y_1} \xi, \map {y_2} \xi}\) | \(=\) | \(\ds \map {y_1} \xi \map { {y_2}'} \xi - \map {y_2} \xi \map { {y_1}'} \xi\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds 0 \cdot \map { {y_2}'} \xi - 0 \cdot \map { {y_1}'} \xi\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds 0\) |
From Zero Wronskian of Solutions of Homogeneous Linear Second Order ODE:
- $\forall x \in \closedint a b: \map W {\map {y_1} \xi, \map {y_2} \xi} = 0$
and so from Zero Wronskian of Solutions of Homogeneous Linear Second Order ODE iff Linearly Dependent:
- $y_1$ and $y_2$ are linearly dependent.
$\blacksquare$
Sources
- 1972: George F. Simmons: Differential Equations ... (previous) ... (next): $\S 3.14$: Problem $7$