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Mathematica shuts down when calculating this limit

Mathematica Asked by An old man in the sea. on December 1, 2020

Bug introduced in 9.0.0 and fixed in 9.0.1


I’m trying to compute this limit:

Limit[E^-(r^2)^(2*n)*r*2*Pi, n -> Infinity]

But, when I run the code above, Mathematica shuts down. I get a warning from Windows stating that the Mathematica 9 Kernel has stopped working. Mathematica itself gives no warning or message. Why is that?

One Answer

With

 $Version
 (* 10.3.0 for Microsoft Windows (64-bit) (October 9, 2015) *)

Mathemeatica returns a limit that depends on r, as it should.

Plot[Limit[E^-(r^2)^(2*n)*r*2*Pi, n -> Infinity], {r, -2, 2}, AxesLabel -> {r, Lim}]

enter image description here

Addendum

Although Limit returns unevaluated for arbitrary r, it can be made to produce useful results by making assumptions on r. For instance,

Piecewise[{Assuming[{#}, Limit[E^-(r^2)^(2*n)*r*2*Pi, n -> Infinity]], #} & /@ 
    {r < -1, r == -1, -1 < r < 1, r == 1, r > 1}]
(* Piecewise[{{0, r < -1}, {(-2*Pi)/E, r == -1}, {2*Pi*r, -1 < r < 1}, 
   {(2*Pi)/E, r == 1}}, 0] *)

which reproduces the plot above, apart from the discontinuities at r = -1 and r = 1.

Correct answer by bbgodfrey on December 1, 2020

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