Plot derivative of an ABS( function ) fails (RESOLVED) - Printable Version +- HP Forums ( https://www.hpmuseum.org/forum)+-- Forum: HP Calculators (and very old HP Computers) ( /forum-3.html)+--- Forum: HP Prime ( /forum-5.html)+--- Thread: Plot derivative of an ABS( function ) fails (RESOLVED) ( /thread-3725.html) |

Plot derivative of an ABS( function ) fails (RESOLVED) - StephanP - 04-27-2015 12:29 PM
While testing the HP Prime I ran into the following odd behaviour. Symb F1(X)=|X^2 -10X +21| (between absolute brackets) F2(X)=dF1(X)/dX=X (the derivative of F1) Plot F1 plots fine, bouncing nicely at x=3 and x=7 F2 does not plot well. It gives three horizontal line segments. Y=1 for x < 3 Y=-1 for 3 < x < 7 Y=1 for x > 7 [attachment=1916] If I use the function Slope in the plot windows, then correct slope values are given for F1. For F2, slope values are "NaN". When I remove the absolute brackets from F1, F2 plots perfectly. RE: Plot derivative of an ABS( function ) fails - StephanP - 04-28-2015 12:25 PM
No response yet. Still I'd like to know. - Is it a bug? - Am I wrong to expect a proper derivative plot? RE: Plot derivative of an ABS( function ) fails - rprosperi - 04-28-2015 01:28 PM
(04-28-2015 12:25 PM)StephanP Wrote: No response yet. It has been less than a full day since your initial post. Please be patient for a reply, many users do not check in hourly or even daily. It seems your interest is more curiosity than some urgent need, so please continue exploring it yourself in parallel; you may find an answer before another user gets to reply. RE: Plot derivative of an ABS( function ) fails - ww63 - 04-28-2015 01:58 PM
Hello, probably the problem is a result of the fact, that the ABS-Function leads to a not continously differntiable function. At the roots of the basic function the ABS-function creates a discontinuity! What is the slope at x = 3 or x = 7? I think the value of the derivative at this points represented by NaN (not a number). regards Wolfgang RE: Plot derivative of an ABS( function ) fails - CR Haeger - 04-28-2015 02:19 PM
F2(X)= SLOPE(F1,X) seems to work. Similar topic Here RE: Plot derivative of an ABS( function ) fails - Tim Wessman - 04-28-2015 02:27 PM
For the moment, you'll need to use that SLOPE command as shown in the other thread. It *was* an issue. :-) RE: Plot derivative of an ABS( function ) fails - StephanP - 04-28-2015 03:58 PM
Yes, SLOPE(F1,X) works. I did find that other thread, but as I was experiencing crashes using slope() (lowercase from the Catalog), I went for the d/dx method. This seemed to work fine, apart from the ABS() case. Still it's a bit confusing, these variants of slope() and SLOPE(). I had to type it manually. Thanks CR, Tim. RE: Plot derivative of an ABS( function ) fails (RESOLVED) - Tim Wessman - 04-28-2015 04:45 PM
(04-28-2015 03:58 PM)StephanP Wrote: I had to type it manually. TOOLBOX->APP->FUNCTION RE: Plot derivative of an ABS( function ) fails (RESOLVED) - CR Haeger - 04-28-2015 04:54 PM
(04-28-2015 03:58 PM)StephanP Wrote: Yes, SLOPE(F1,X) works. Toolbox > App > Function > SLOPE is what I use while in the Function App. Another interesting way to key this into CAS and send to Function App: f(x):= |X^2 -10X +21| F1:=f F2:=f' which defines f(x), F1 as f(X) and F2 as f'(X). Hit Plot and it should display ok. |