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 Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp bo198214 Administrator Posts: 1,389 Threads: 90 Joined: Aug 2007 06/03/2008, 08:09 AM (This post was last modified: 06/03/2008, 08:15 AM by bo198214.) andydude Wrote:So, not only can we show that f(x) is a constant, but we can show it in multiple ways, which is a very bad sign. I think this is proof that no such function can exist (or the only solution to all of these equations is f(x)=1). Yeah, I see its even much simpler to prove than I did, and we can also drop the demand of continuity. However your proof(s) omits the case of $f(x)=0$. So I will add the detail: If $f(x^y)=f(x)f(y)$ for f being defined on $[1,\infty)$, then $f(x)=f(x)f(1)$ for $y=1$. So if there is at least one $x$ with $f(x)\neq 0$ (i.e. $f$ is not identically 0) then $f(1)=1$. Let now $x=1$ in our original equation: $f(1)=f(1)f(y)$. With $f(1)=1$ we get $f(y)=1$ for all $y$. So either $f=1$ or $f=0$. @Ivars: this proof also works for hyperreals. « Next Oldest | Next Newest »

 Messages In This Thread Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by Ivars - 06/01/2008, 01:54 PM only identity maps powers to products - by bo198214 - 06/01/2008, 03:01 PM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by Ivars - 06/01/2008, 05:02 PM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by andydude - 06/02/2008, 04:46 PM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by bo198214 - 06/03/2008, 08:09 AM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by Ivars - 06/03/2008, 08:49 AM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by bo198214 - 06/03/2008, 01:13 PM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by Ivars - 06/03/2008, 01:22 PM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by bo198214 - 06/03/2008, 03:21 PM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by Ivars - 06/02/2008, 07:15 PM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by Ivars - 06/03/2008, 07:14 AM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by Ivars - 06/03/2008, 03:34 PM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by bo198214 - 06/03/2008, 05:14 PM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by Ivars - 06/03/2008, 09:45 PM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by andydude - 06/04/2008, 01:51 AM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by Ivars - 06/04/2008, 07:20 AM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by bo198214 - 06/04/2008, 02:14 PM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by Ivars - 06/04/2008, 02:33 PM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by BenStandeven - 05/12/2009, 01:00 AM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by Ivars - 12/04/2008, 01:15 PM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by JmsNxn - 04/14/2011, 11:16 PM RE: Attempt to formally generalize log, exp functions to 3,4,5..(n,m) log exp - by bo198214 - 04/15/2011, 07:29 AM

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