The "notational second derivative" is true only when the function 𝑓(𝑥) is of a very specific form
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Yeh but f(x)=2 is also a solution but not covered by this, right?
absolutely it is.. this is the most general solution to the 2nd order ode, and you can certainly find initial conditions to make it a "particular" solution, which is what you're asking for..
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