I thought I figured out what I was looking for.

The points of discontinuity occur where the slope of the rotated curve
is vertical, which corresponds to the zeros of the derivative if I rotate
my curve an additional 90 degrees.

discon=: (#~ 0 = {:"1@:+.)@(1 {:: p.)@((}.@:* [EMAIL PROTECTED])@] -/@,: 
0,3&[EMAIL PROTECTED]@[)

Unfortunately, this is not very efficient, nor does it produce a viable
result.

   p=: ($&0 1 0 _1 % [EMAIL PROTECTED]) 30
   0.375 discon p   NB. very slow
19.2422 _19.2422

But from inspection, I have three discontinuities between _9 and 9.
And I expect them to correspond to my control parameter being between...
   dot=: +/ .*
   rot=: (1 _1,:1 1) * (2 1,:1 2)&[EMAIL PROTECTED]
   ({~ (0 1 +/~0 1 [EMAIL PROTECTED] (] = >./\)@{.)@((rot 0.375)&dot)@(,: 
p&p.)) 0.1*i:90
_3.8 _3.7
 2.4  2.5
 8.7  8.8

So I still have not figured this out.

-- 
Raul
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