In the following, the diff operator calcuates the derivative correctly, but the D operator doesn't.  A bug?

Here is a worksheet containing the commands above in case you want to try it yourself: mw.mw

In the (r,t) polar coordinates, the graph of the equation r = 1/cos(t) should be a straight vertical line segment but Maple produces junk.  What is going on?

plot(1/cos(t), t=0..Pi/3, coords=polar);

The parametric version

plot([1/cos(t), t, t=0..Pi/3], coords=polar);

produces the same thing.

Is there a way to tell Maple to export a figure to PDF with a proper bounding box, like all of its other graphics export formats?

To export a graphics produced by Maple's plot(), I right-click on the plot, select Export, and then one of the several choices of graphics formats.  All options, other than the PDF, work fine—they produce graphics files whose bounding boxes correspond to the extents of the image.  Exporting to PDF misbehaves—it produces the equivalent of a 8.5''x11'' paper and inserts the graphics somewhere near the upper left corner.  I am absolutely at a loss to see the utility of that.  What in the world is the use of such an export?

Is there a configuration setting that tells Maple to save to PDF with a proper bounding box?  I looked around but couldn't find one.

In the attached Maple worksheet I attempt to plot the solution of an initial value problem for a first order ODE.  DEplot fails with a cryptic message.  Strangely enough, if I give the "arrows=none" option to DEplot, it produces the correct plot!

I see this behavior in Maple 17 and 18.

Maple 11, however, works fine with or without the "arrows=none" option.

Is there an explanation for this or is it a bug?

DEplot-bug.mw

The following works as intended:

restart;
diff(x^7, x$i):
seq(%, i=0..7);

Combining the two commands into one, however, does not work:

restart;
seq(diff(x^7, x$i), i=0..7);

          Error, invalid input: diff expects 2 or more arguments, but received 1

How does one explane this?  I was unable to find the reason by looking at the help page for seq().