## 273 Reputation

19 years, 323 days

## VectorCalculus:-DirectionalDiff problem...

Maple

I was setting up a worksheet for my vactor calculus class and tried to show how the DifferentialDiff command from the VectorCalculus package was limited.  I was expecting to have it give undefined or something similar for all directions with the function I am using.  Unfortunately, I got 0 when the directional derivative is truly undefined and I got undefined when the directional derivative is 0.

This is the code I am using.  Unfortunately, this site will not take the worksheet or plot the function for me.

with(VectorCalculus):
s := min(abs(x),abs(y));
plot3d(s,x=-1..1,y=-1..1);
DirectionalDiff(min(abs(x),abs(y)),<-1,1>,[x,y],point=<0,0>);

## Why does SurfaceInt in spherical coordin...

Maple 2020

A person asked me why the following code does not give the integral of (x^2+y^2)*z over the top of the unit sphere.

```VectorCalculus:-SurfaceInt((x^2 + y^2)*z, [x, y, z] = Surface(<1, s, t>, s = 0 .. Pi/2, t = 0 .. 2*Pi, coords = spherical))
```

The correct answer is Pi/2, but Maple gives 2*(-1 + Pi)*Pi^2.

Will someonoe please explain this behavior?

## The plots:-fieldplot grid option can giv...

I was playing with plots:-fieldplot in Maple 2019 and found that sometime the grid option gives a strange result.  The code I used was

```with(plots): fieldplot([-x/(x^2+y^2)^1.5,-y/(x^2+y^2)^1.5],x=-1..1,y=-1..1); fieldplot([-x/(x^2+y^2)^1.5,-y/(x^2+y^2)^1.5],x=-1..1,y=-1..1,grid=[8,8]); fieldplot([-x/(x^2+y^2)^1.5,-y/(x^2+y^2)^1.5],x=-1..1,y=-1..1,grid=[7,7]);```

The first two plots come out as expected.  The third plot has (0,0) in the grid, a point where the vector field does not exist.  Rather than giving an error or ignoring the point, Maple 2019 (and 2018) plots a single arrow through the origin. The file with plots is below.  I hope that this can be changed to give a different output.

 > with(plots): fieldplot([-x/(x^2+y^2)^1.5,-y/(x^2+y^2)^1.5],x=-1..1,y=-1..1); fieldplot([-x/(x^2+y^2)^1.5,-y/(x^2+y^2)^1.5],x=-1..1,y=-1..1,grid=[8,8]); fieldplot([-x/(x^2+y^2)^1.5,-y/(x^2+y^2)^1.5],x=-1..1,y=-1..1,grid=[7,7]);
 >

## PNG not listed in plotting devices help ...

Maple 17

Does anyone know why png is not listed in the help on plot/devices?  The png device exists and works.

Also, are there any other unlisted plot devices?

## How do you show the student's name?...

I have a test server up for MapleTA 8.  Where is the option to show a student's name on the questions?

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