# Infinite Series

1. Aug 21, 2009

### Gear300

I just need to make sure that I've got this analysis right:

The argument S = 1 - 1 + 1 - 1 + 1 - ...
then S = (1 - 1) + (1 -1) + (1 -1) + ... = 0 is invalid because it ignores all sum Sn for n not congruent modulo 2 (not even).

The argument S = 1 - 1 + 1 - 1 + 1 - ...
then S = 1 - (1 -1) - (1 - 1) - ... = 1 is invalid because it ignores all sum Sn for n congruent modulo 2 (even).

The argument S = 1 - 1 + 1 - 1 + 1 - ...
then S = 1 - (1 - 1 + 1 - 1 + ...) = 1 - S,
S = 1 - S
S = 1/2 is invalid because S + S (from adding the terms) is the same as S. From this, it seems as though algebraic operations such as addition and multiplication by scalars do not seem to work the same way with infinite series as they do with numbers.

The argument S = 1 + 2 + 4 + 8 ...
Then 2S = 2 + 4 + 8 + 16 ...
2S = S - 1
S = -1 is invalid because (from the third argument) 2S - S is not necessarily S.

What struck me as odd was that even though infinite series, such as these, consist of algebraic operations on Real Numbers (integers in this case), algebraic operations do not seem to work on them in the same way...why is that so?

2. Aug 21, 2009

### CRGreathouse

Because the series diverges. You can use those rules on series that converge absolutely, but not (without Caesaro, etc. summation) on divergent series.

3. Aug 21, 2009

### Gear300

I see...are there defined operations for diverging series?

4. Aug 21, 2009

### g_edgar

There are whole BOOKS on the subject ... such as:

G. H. Hardy, Divergent Series, 1929

5. Aug 21, 2009

### Gear300

I see...thanks for the replies.

6. Aug 21, 2009

### Elucidus

Even when the series is conditionally convergent, such as the harmonic series $\sum_{n=1}^{\infty} \frac{1}{n}$, there are still problems with associativity and commutativity of addition and distribution.

There is a theorem (which may be called Riemann's Rearrangement Theorem) that says for any conditionally convergent series there exists a permutation of the terms that will converge to any finite number desired or diverge to infinity or minus infinity.

The only series where the normal algebraic properties hold are those that are absolutely convergent.

--Elucidus

7. Aug 22, 2009