What does $u_{xx}$ and $u_{tt}$ mean?

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I was reading this answer; it dealt with wave-equation; though I know the D'Alembert formula but I'm new to this notation.

What does $$u_{xx}= u_{tt}$$ and $$v_{xx}= v_{rr}$$ mean?

Sorry, if this is a dumb question but I'm new to this type of notation.

So, could anyone please help me conceive this notation?

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2 Answers

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subscript is a derivative with respect to this variable.

$$ u_{xx} = \frac{\partial^2u}{\partial x \partial x} $$

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$u$ is a two variable function $u(x,t)$,

$$u_{xx}=\frac{\partial^2x}{\partial x^2}$$.

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