plotting problem - empty graph

Questions
1

f[x_] = ((x - 50000)^4.51846) (E^(50000 - x)) (4.6924*^-7) - this is my function.
Plot[{f[x]}, {x, 0, 50000}] command gives me no GRAPH!!!
What should i conclude?
When i want to see the data points using “data = (InputForm@p)[[1, 1, 1, 3, 2, 1]]”.
I am not able to interpret the result!! something like this comes up…

Graphics[GraphicsComplex[list of points…]
Can anybody throw some light on this?

2

I do not know why it returned GraphicsComplex instead of Line here. btw, you need to use := not = in the function definition. When asking questions, it helps to post complete self contained code you used all in one cell. This helps someone see what you actually typed. Include all definitions and commands used.

3

This was the code i used for plotting f[x] versus x.
f[x_] := ((x - 50000)^4.51846) (E^(50000 - x)) (4.6924*^-7)
After typing this, when i entered f[x]
the result was
4.6924*10^-7 E^(50000 - x) (-50000 + x)^4.51846
It means the function definition is correct. After this when i wanted to plot f versus x,
i used the following command

p= Plot[f, {x, 0, 50000}]
I did not get any graph!!!
i wanted to know the points. For that i used
“data = (InputForm@p)[[1, 1, 1, 3, 2, 1]]”.
After this the result was,
Graphics[GraphicsComplex[{{0.00010204081632653062, 0.}, {98.143306007652, 0.},
{204.54178542729326, 0.},etc…
Now
Can anyone explain what i should do? I am not able to understand this..

4

Hi,

there are several “issues” here. First of all

(x - 50000)^4.51846

is complex for x<50000, e.g.

(x - 50000)^4.51846 /. x -> 2
is -9.88935*10^19 + 1.70333*10^21 I.

Plot does not plot complex numbers.The "interesting part of your graph is for x>50000.

Also, you should plot f[x] instead of “f”.

f[x_] := ((x - 50000)^4.51846) (E^(50000 - x)) (4.6924^-7)
Plot[f[x], {x, 50000, 50050}, PlotRange -> All]

does work and gives a reasonable output:

fcommunity.jpg
fcommunity.jpg360×208 6.35 KB

So, if you plot it for x< 50000 it will not produce a useful output, because the Plot function is not made to plot complex functions.

Furthermore, the values that you want to plot are insanely large. Look at this:

Table[Evaluate[f[x]], {x, 0, 5}]

{-1.046106944790515*10^21730 + 
  1.801802763355185*10^21731 I, -3.848064616838928*10^21729 + 
  6.627862949115713*10^21730 I, -1.415495933981097*10^21729 + 
  2.438034178117325*10^21730 I, -5.206847948258534*10^21728 + 
  8.968215982373779*10^21729 I, -1.915319207666480*10^21728 + 
  3.298924128423410*10^21729 I, -7.045428821964528*10^21727 + 
  1.213496687279907*10^21729 I}

In the interesting regime 50000<x<50050 the values are very small. It will be difficult to have a reasonable representation over the entire domain.

Cheers,
Marco

5

Same issue as reasonable function definition gives empty plot

Be aware that clever things like

data = (InputForm@p)[[1, 1, 1, 3, 2, 1]]

are not guaranteed to be the same in different versions.