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Re: mg[14566]: Follow-up on Piecewise Continuous Functions

  • To: mathgroup at
  • Subject: [mg14587] Re: mg[14566]: Follow-up on Piecewise Continuous Functions
  • From: Jack Goldberg <jackgold at>
  • Date: Mon, 2 Nov 1998 01:50:59 -0500
  • Sender: owner-wri-mathgroup at

Hi Group:

I remarked in my earlier post that manipulating PC functions  was not
difficult and I promised to show why and how in  subsequent posts. 
Here I explain the basic ideas. 

(1).  Let us agree on a standard form for PC functions. I prefer using
Which[]. Examples:

tent[x_] := 
	Which[ -Infinity<x<0, 0, 0<=x<1, x, 1<=x<2, 2-x, 2<=x<Infinity, 0 ]   

square[x_] := 
	Which[ -Infinity<x<0, 0, 0<=x<1, 1, 1<=x<Infinity, 0 ]

noname[x_] := 
	Which[ -Infinity<x<0, x, 0<=x<Infinity, 1 ]

(a)  All functions are defined for all x.  If necessary we use 
"-Infinity < x < a, 0"   and   "b<= x < Infinity < 0"  to insure  that
the functions are defined everywhere.  

(b)  The functions are continuous from the right, that is, 
 	f(a) = Lim f(x), x->a, x>a

(c)  In  "Which[..., a<=x<b, f(x), ...]"  f(x) is assumed to  be
analytic in  a<x<b.  (So  f[x]  is not itself a piecewise continuous

Now the question.  The function 

(A)	noname[x] + tent[x]*square[x]

for example, is a PC function.  What is its canonical Which form? To
construct the which form for this function, I rely on a new function
called "step".  (Not to be confused with UnitStep[]  which appears in 
many packages but is not built-in at least in Version 3.0 and earlier.)

(2).  Defining "step"

step[x_, a_, f_]  is not available to the user.  It is a function  used
only in the private part of the package as an intermediate 
computation.  It is a numerical function of  x, a  and  f(x) but is
never evaluated.  Its meaning is this:

step( x, a, f(x) )  returns 0 if x<a, returns  f(x)  if  x>=a. 

Various simplification rules are defined for "step"  (as of now  I have
9) Here are some samples.

step  /:  step[ x_,a_,f_ ]*step[ x_, b_, g_ ] := step[ x,Max[a,b], f*g ]
step /:   step[ x_,a_,f_ ]^n_Integer?Positive := step[ x,a,f^n ]

step /:   step[ x_,a_,f_ ]+step[ x_,a_,g_ ] := step[ x,a,f+g ]

(3).  The central idea.

The private function  "tostep[]"  takes as an argument any combination 
of canonical Which[]'s  such as the one given in (A) above, and
produces  a sum of step functions.  The private function  "fromstep[]"
converts output of "tostep' to the canonical  "Which[]".  The rules
that define "step" when used in "tostep" always lead to a sum of this

step[ x,a1,f1[x] ] + step[ x,a2,f2[x] ] + ... + step[ x,an,fn[x] ]

where  a1 < a2 < ... < an.  From such sums,  "fromstep[]"  easily 
converts to a single canonical  "Which[]".  Handling ordinary functions
combined with PC functions is easy, since a separate program writes 
f(x)  as  Which[ -Infinity<x<Infinity,f]  or, if necessary,
 	Which[ -Infinity<x<a, 0,  a<=x<b, f(x), b<=x<Infinity, 0 ] 

This latter case occurs for functions such as  Sqrt[1-x^2]  which is 
not defined for  |x|>1.  

The actual implementation of each of these functions (step, tostep,
fromstep, etc)  is not difficult but is a bit tiresome.  The code
itself is less than 100 lines and has printing and tabulation options
that  make for easier visualization.  For example, the user function 


calls on the private functions to return the canonical PC representation
for any (?)  f(x).  An optional to  ToPiecewise  sketches the graph; a 
second option writes the PC function in a convenient  ColumnForm.

The code will be posted (I hope!) next week.


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