Wednesday, 10 December 2014

calculus - A function with certain shape



I'm trying to simulate a certain type of condition with a continuous and twice differentiable function f(x) that has the following shape:





  1. The limit of f(x) at is finite but less or equal to zero, i.e.
    <limxf(x)0

  2. There exists a unique point yR such that f(x) is decreasing on (,y) and increasing on (y,).

  3. The limit at is positive or infinity i.e. 0<limxf(x).



However I'm struggling to find a good example of such a function that is simple enough to make the point clear. In other words I'm looking for a explicit examples of a function with the properties that I described above.


Answer




Here's one that will work:
f(x)={ex2,x<012ex2/2,x0.


Here's a plot:



enter image description here



Here's another that might work (inspired by David G. Stork's answer):
f(x)=tanh(x)ex2/4.


Plot:




enter image description here



The thing is, I haven't double-checked that this function is monotonically decreasing before some x, and monotonically increasing after.


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