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Im looking at the boundary value problem $$\Theta''+\lambda\Theta=0$$ with periodic boundary conditions $\Theta(\theta)=\Theta(\theta+2\pi)$ and $\Theta'(\theta)=\Theta'(\theta+2\pi)$. For the case when $\lambda < 0$, setting $\alpha=\sqrt{-\lambda}$, we get a general solution $$\Theta(\theta)=A\cosh(\alpha\theta)+B\sinh(\alpha\theta)$$ for constants $A,B$ and so $\Theta'(\theta)=A\alpha\sinh(\alpha\theta)+B\alpha\cosh(\alpha\theta)$. The boundary conditions then imply that $$A\cosh(\alpha\theta)+B\sinh(\alpha\theta)=A\cosh(\alpha(\theta+2\pi))+B\sinh(\alpha(\theta+2\pi))$$ $$A\alpha\sinh(\alpha\theta)+B\alpha\cosh(\alpha\theta)=A\alpha\sinh(\alpha(\theta+2\pi))+B\alpha\cosh(\alpha(\theta+2\pi))$$ How does this imply only the trivial solution, that $A=B=0$?

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  • $\begingroup$ Try plugging in $\theta = 0$ (and maybe $\theta = -2\pi$) and solving the system of equations. $\endgroup$ Commented Apr 23, 2016 at 2:42
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    $\begingroup$ @CameronWilliams : you need exploiting $\Theta(0) = \Theta(2\pi)$ and $\Theta'(0) = \Theta'(2\pi)$, both conditions tell us that the only solution is $\Theta(\theta) = 0$ for $\Theta''(\theta)+\lambda \Theta(\theta) = 0$ with $\lambda < 0$ $\endgroup$
    – reuns
    Commented Apr 23, 2016 at 2:48
  • $\begingroup$ @user1952009 I meant into both equations, not just one. $\endgroup$ Commented Apr 23, 2016 at 2:49
  • $\begingroup$ @CameronWilliams : since the solutions are of the form $\Theta(\theta) = a e^{\sqrt{-\lambda} \theta} + b e^{-\sqrt{-\lambda} \theta}$, two constants, two equations needed $\endgroup$
    – reuns
    Commented Apr 23, 2016 at 2:50
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    $\begingroup$ @user1952009 Yes that is what I was saying. I was not disagreeing with you, merely clarifying what I meant. $\endgroup$ Commented Apr 23, 2016 at 2:51

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  1. Assume that $\Theta$ is twice differentiable, periodic, and not identically zero.

  2. Multiply OP's ODE $$\Theta^{\prime\prime} +\lambda\Theta ~=~0 \tag{1}$$ with $-\bar{\Theta}$ on both sides and integrate over a period: $$ 0~=~-\int_0^{2\pi}\! d\theta \left( \Theta^{\prime\prime} +\lambda\Theta \right)\bar{\Theta}~\stackrel{\text{int. by parts}}{=}~\underbrace{\int_0^{2\pi}\! d\theta |\Theta^{\prime}|^2}_{\geq 0}-\lambda\underbrace{\int_0^{2\pi}\! d\theta|\Theta|^2}_{> 0} . \tag{2}$$ The integration by parts yields no boundary terms since $\Theta$ is assumed to be periodic.

  3. Conclude that $\lambda\geq 0$ is non-negative.

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  • $\begingroup$ Good. I got the sign wrong in my comment... $\endgroup$
    – mickep
    Commented Apr 23, 2016 at 14:13

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