Monad in non-standard analysis

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Discussion Overview

The discussion revolves around the concepts of infinitesimals in the hyperreals and the definition of monads in non-standard analysis. Participants explore whether 0 is considered an infinitesimal and whether a monad includes real numbers, referencing various sources and definitions.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants question whether 0 is considered an infinitesimal in the hyperreals, with conflicting initial responses.
  • One participant cites a Wiki page that defines a monad with respect to a real number x, suggesting that if 0 is not an infinitesimal, then x would not be included in the monad.
  • Another participant references a source indicating that "the only real infinitesimal is 0," leading to further clarification about the nature of infinitesimals.
  • There is a discussion about the distinction between "real" and "Real" infinitesimals, with some participants seeking clarification on this terminology.

Areas of Agreement / Disagreement

Participants express differing views on whether 0 is an infinitesimal, and there is no consensus on the definitions being used. Some participants retract their earlier statements based on new information, but disagreement remains regarding the implications of these definitions.

Contextual Notes

There are references to specific definitions and sources that may not be universally accepted, and the discussion reflects varying interpretations of the terms involved.

nomadreid
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Quick two questions:
(a) In the hyperreals, is 0 considered an infinitesimal?
(b) Does a monad include the real number?
I seem to get contradictory answers in the Internet.
Thanks.
 
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nomadreid said:
(a) In the hyperreals, is 0 considered an infinitesimal?
No. Reconsidering, I'll say yes.

I don't know the answer to your b part. Can you provide more information
 
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In the Wiki page on monads
https://en.wikipedia.org/wiki/Monad_(nonstandard_analysis)
it defines the monad with respect to a real number x as anything is difference to x is infinitesimal. To me, that would exclude x (since, as you wrote, 0 is not an infinitesimal).
Yet is also says
"the unique real number in the monad of x is called the standard part of x"
Which would seem to imply that r was in the monad.
So what am I reading incorrectly here?
Thanks.
 
Thank you, Frabjous. Accordingly, 0 is an infinitesimal, so the monad definition in Wiki works after all .
Mark44 -- you answered that 0 is not an infinitesimal; are there two different definitions to be found?
 
nomadreid said:
Mark44 -- you answered that 0 is not an infinitesimal; are there two different definitions to be found?
No. After reading the first few pages of the link that @Frabjous provided, I retract my earlier response. From that source, "the only real infinitesimal is 0."
 
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Mark44 said:
No. After reading the first few pages of the link that @Frabjous provided, I retract my earlier response. From that source, "the only real infinitesimal is 0."
The only real or Real infinitesimal?
 
WWGD said:
The only real or Real infinitesimal?
The only infinitesimal that happens also to be real. Again, according to the paper/book by Keisler.
 
Mark44 said:
The only infinitesimal that happens also to be real. Again, according to the paper/book by Keisler.
Not doubting you, just curious as to what you meant.
 

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