Re: Re: Re: Bug in interpretation of mma Series[] command?

• To: mathgroup at smc.vnet.net
• Subject: [mg2343] Re: [mg2287] Re: [mg2268] Re: Bug in interpretation of mma Series[] command?
• From: Richard Mercer <richard at seuss.math.wright.edu>
• Date: Wed, 25 Oct 1995 02:13:35 -0400

```Mgroupers,

A couple lines were inadvertantly omitted from my previous poston this
subject. Following is the correct version.
In essence I agree with Dave Withoff and Paul Abbott, but my point here
is that many users feel the other way, and not unreasonably.

Richard Mercer
---------------------------------------------------------------------
In view of Dave Withoff's response to this issue, perhaps I was a

His reply was cogent as always and from a system design viewpoint is

definitely the right approach.

However the best system design is not always the best for the user.
The ideal situation for many users is for

Exp[-a y^2] * Series[1/(1 + y^2), {y,0,5}]

to act like

Exp[-a y^2] * Normal[Series[1/(1 + y^2), {y,0,5}]]

Reasons:
(1) Most average (nonexpert) users do not have a mental model

corresponding to a SeriesData object; they think of the result of a

Series command as being a polynomial and expect it to behave that way

in calculations.
(2) The structure Series[Exp[-a y^2] * 1/(1 + y^2), {y,0,5}] is

available and much more natural if you want the exponential converted

to a series.

The best solution would seem to be a user-settable "switch" that would

apply Normal to the output of all Series commands. This would

presumably satisfy all those who expect Series objects to act like

polynomials in this and other situations.

One possible way to do this, though far from perfect, is to use

\$Post = (# /.

Literal[SeriesData[stuff__]]:>Normal[SeriesData[stuff]])&;

(You could put this in your init.m file if you really like it.)
Two problems with this:
(1) It can potentially corrupt some things you don't want converted.
(For example, its own output! Try removing the semicolon.)
(2) More importantly, it's an expert solution to a non-expert problem.
The people who can/will do things like this are pretty much the people

for whom it's not necessary.

```

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