Series Expansion Inconsistency

• To: mathgroup at yoda.physics.unc.edu
• Subject: Series Expansion Inconsistency
• From: Cetin Cetinkaya <cetin at acm0.me.uiuc.edu>
• Date: Mon, 19 Apr 1993 20:19:27 -0500

``` Hello;

While I am trying to do some perturbation analysis, I run into a peculiar
behaviour. My function k looks like

In[31]:= k
0. Second  1928.568 KByte

3 - n                  3 - n
(c1 eps)      - (c20 + c21 eps)
Out[31]= -------------------------------------------------------------
2 - n                  2 - n
c1 eps (c20 + c21 eps) ((c1 eps)      - (c20 + c21 eps)     )

For n=19, I expand the function k with respect to eps up to eps^2 and
collect the coefficients of eps:

In[32]:= Collect[Simplify[Normal[Series[k/.n->19,{eps,0,2}]]] ,eps]
3.15 Second  1929.216 KByte

1
Out[32]= ---
c20

This simply means that the expansion does not have eps or eps^2 orders
terms. However, When I tried the following to see how the eps^3 order
term looks like, I have got this:

In[33]:= Collect[Simplify[Normal[Series[k/.n->19,{eps,0,3}]]] ,eps]
3.81667 Second  1930.156 KByte

2    2      3    3
1    c21 eps   c21  eps    c21  eps
Out[33]= --- - ------- + --------- - ---------
c20       2          3           4
c20        c20         c20

To my suprise, this expression has order eps and eps^2 terms. Could any
one explain what is going on?
The Mathematica verson I am running is 2.1.

Cetin

```

• Prev by Date: Question about Display
• Next by Date: Mathieu