Problem plotting a function
10-05-2015, 02:03 AM
Post: #1
 Brad Barton Member Posts: 194 Joined: Jan 2014
Problem plotting a function
I was going over some algebra homework with my oldest daughter tonight, and was showing her some graphs on the Prime to help her understanding. Also it was an opportunity to show off my new toy so there's that.

Anyway, using the Function app, I showed her a plot of y=X^6-8X^5+12X^4. We'd factored it and found that it has roots at 0, 2 and 6. We were surprised to see that the Prime missed plotting the root at 6. I suppose that the slope of the graph is so steep in this area that it looks like a discontinuity to the Prime.

I checked the manual and it doesn't list any limitations for simple polynomial functions for the Function app. The equation plotted correctly in the Advanced Graphing app, but it took a while to narrow down the answer.

Is there some setting somewhere that tells the Prime what constitutes a discontinuity? Is there a listing of the function types suited to each of the graphing apps?
10-05-2015, 02:57 AM (This post was last modified: 10-05-2015 02:59 AM by eried.)
Post: #2
 eried Senior Member Posts: 744 Joined: Dec 2013
RE: Problem plotting a function
I think it could be related with the zoom, if you set it to be very far away in the Y axis you see this:

Probably the way the prime joins the dots evaluating the function aborts the drawing on these cases.

My website: erwin.ried.cl
10-05-2015, 03:03 AM
Post: #3
 John Colvin Member Posts: 173 Joined: Dec 2013
RE: Problem plotting a function
That's strange. It plots the same way on my Prime using the Function app. It plots
correctly on my 50G. Might be a bug.
10-05-2015, 03:49 AM
Post: #4
 ColinJDenman Member Posts: 102 Joined: Feb 2014
RE: Problem plotting a function
It is really a 'feature' of the plotting system. It happens frequently. You may find that changing the plot settings method gives you better mileage. Usual default tends to be adaptive, but you may see what you expect with one of the fixed step plotting methods. Adaptive tries to be clever but sometimes fails.
10-05-2015, 05:44 AM
Post: #5
 cyrille de brĂ©bisson Senior Member Posts: 1,047 Joined: Dec 2013
RE: Problem plotting a function
Hello,

The Prime plotting engine 'tries' to provide a 'good' display of the function.
For example, it attempts to detect discontinuities and the like correctly.

In this case, with the plot basic setting, the function that you are trying to plot does oscillate so fast that it does look like a discontinuity from the point of view of the "imprecise" floating point math library (ie, not exact) and it is displayed as such.

Zooming out fixes this issue.

The advanced grapher does NOT have this problem and will draw the function properly (but much slower)

Cyrille

Although I work for the HP calculator group, the views and opinions I post here are my own. I do not speak for HP.
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