1 + 10 + 100 + 1000 + = -1/9

  • Context: Undergrad 
  • Thread starter Thread starter Unit
  • Start date Start date
Click For Summary

Discussion Overview

The discussion centers around the summation of the series S = 1 + 10 + 100 + 1000 + ..., exploring its implications and the validity of deriving S = -1/9 through various mathematical approaches. Participants examine the convergence of the series in different number systems, including p-adic fields, and the legitimacy of manipulating infinite sums.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents a method to derive S = -1/9 using algebraic manipulation of the series.
  • Another participant references Ramanujan summation as a potential framework for understanding the series.
  • Some participants discuss the convergence of the series in p-adic fields, noting that it converges in 2-adics and 5-adics but not in the reals.
  • There is a contention regarding the convergence of terms in other p-adic fields, with some asserting that terms do not converge to zero.
  • Concerns are raised about the validity of pairing terms in an infinite sum, suggesting that if S does not exist, the derived statements may not hold.
  • A participant highlights the importance of hypothetical reasoning in the context of the existence of S.

Areas of Agreement / Disagreement

Participants express differing views on the convergence of the series in various mathematical contexts and the validity of the manipulations used to derive S = -1/9. The discussion remains unresolved regarding the implications of these manipulations and the nature of convergence in different number systems.

Contextual Notes

Limitations include the dependence on definitions of convergence in different mathematical frameworks and the unresolved nature of the manipulations applied to the infinite series.

Unit
Messages
181
Reaction score
0
S = 1 + 10 + 100 + 1000 + 10000 + ...

10S = 10 + 100 + 1000 + 10000 + 100000 + ...

S - 10S = (1 + 10 + 100 + 1000 + 10000 + ...) - (10 + 100 + 1000 + 10000 + ...)

-9S = 1 + (10 - 10) + (100 - 100) + (1000 - 1000) + (10000 - 10000) ...

-9S = 1 + 0 + 0 + 0 + 0 + 0 ...

-9S = 1

S = -1/9

:confused:
What's wrong (or right) with this?

Thanks,
Unit
 
Mathematics news on Phys.org
Also, I believe that sum converges as an ordinary infinite sum in the 2-adics and the 5-adics.

(And, of course, it does not converge as an ordinary infinite sum in the reals!)
 
I would have [intuitively] expected it to converge in all the p-adics. Am I wrong?
 
In any other p-adic field, the terms don't converge to zero!
 
Hurkyl said:
In any other p-adic field, the terms don't converge to zero!

:blushing:
 
I'm pretty sure that you can't pair up terms in an infinite sum.
 
Unit said:
S = 1 + 10 + 100 + 1000 + 10000 + ...
.
.
.
What's wrong (or right) with this?

Thanks,
Unit

Char. Limit said:
I'm pretty sure that you can't pair up terms in an infinite sum.
The way I remember it, proofs like this actually say something like:
If S exists, then S = 1 + 10 + 100 + ...​
So if S does not exist, then the remaining statements do not necessarily hold true.
 
  • #10
Redbelly98 said:
If S exists, then S = 1 + 10 + 100 + ...​
So if S does not exist, then the remaining statements do not necessarily hold true.

:smile: Brilliant! I had completely forgotten about variables and their related hypothetical syllogisms. Thanks!
 

Similar threads

  • · Replies 17 ·
Replies
17
Views
2K
  • · Replies 2 ·
Replies
2
Views
1K
  • · Replies 3 ·
Replies
3
Views
913
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 16 ·
Replies
16
Views
1K
  • · Replies 18 ·
Replies
18
Views
5K
  • · Replies 12 ·
Replies
12
Views
1K
  • · Replies 18 ·
Replies
18
Views
4K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 1 ·
Replies
1
Views
2K