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MathGroup Archive 1997

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Exponent function

  • To: mathgroup at
  • Subject: [mg7807] Exponent function
  • From: Raya Firsov-Khanin <raya at>
  • Date: Sat, 12 Jul 1997 02:45:49 -0400
  • Sender: owner-wri-mathgroup at


I've noticed the following strange feature of 

Exponent[expr, arg, List]

function of Mathematica.  Suppose I want to 
find all powers of x in the expression of the type 

test = x + x^(1+a) + x^(-1+a) + x^2 

then Exponent[test, x, List] gives the correct result 

{1, 2, 1-a, -1+a}. 

Suppose now, I add x^(-1) to test:  

test1 = test + x^(-1) 


Exponent[test1, x, List] gives a result {-1, -1, 1, 1, 2}. 

For expressions like 

test2 = test + x^(-n) the result of Exponent[] is correct 

{1, 2, 1-a, -1+a, -n}, while for expressions where n is specified, 
e.g.  n=1, 2, 3, ...  the result is always wrong.

I need to use Exponent function in a program that
should solve nonlinear problems using Multiple Scale Method. I need to
identify exponential frequencies of different terms in the equations, and
to study under what conditions they become resonant.  

Any suggestions on how I can make Exponent function to give correct 
results? Or may be there are some other ways to find powers of x in the 
expressions of the type written above.  
Any help would be appreciated. I already spent a morning trying to solve 
this puzzle.  
		Thanks a lot. 

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