Re: error with Sum and Infinity

• To: mathgroup at smc.vnet.net
• Subject: [mg102280] Re: error with Sum and Infinity
• From: Richard Fateman <fateman at cs.berkeley.edu>
• Date: Tue, 4 Aug 2009 04:26:22 -0400 (EDT)
• References: <h56bq2\$buv\$1@smc.vnet.net>

```You've gotten several of the standard work-arounds or excuses, which are
really  standard issue here.

1. Mathematica is doing the right thing; [by definition] so the mistake
is yours.

2. If you computed something else, different from what you wrote, you
would get a different answer, but corresponding to what you expected.

Here's one more standard, (from me, anyway).   Mathematica has a design
problem.

The underlying point is that Mathematica is conflating two concepts with
the name Sum:

A.  A loop of finitely many terms evaluated in sequence and adding up
the terms.   and

B.  A symbolic calculation based on various combinatorial ideas, the
calculus of finite differences, and other systematic simplifications
that reduces a summation, either finite or infinite, into a result that
does not have any summation notation in it.  Like summing arithmetic
progressions, geometric progressions, etc (and very advanced etc.).

For this second concept to work, the summand must be something that can
be suitably manipulated, typically starting as a single algebraic
expression. A programming segment, or a pattern match that requires that
each value of the index be fed into an evaluator will not, generally
work with algorithms for indefinite or definite/infinite summation.
Obviously you cannot feed an infinite number of index values into a
function and sum up all the terms.

A clean solution would be to separate these two concepts:  a loop and a
symbolic closed-form simplifier for a summation. Or for Mathematica to
use the Sum form, but somehow allow you to indicate to the system that
you want it evaluated as a loop or simplified to a closed form.

RJF

```

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