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using sinh(x) ?
#1
im thinking about using asymt of exp(x) to solve tetration.

perhaps not the best one , but sinh(x) comes to my mind.

if we take the half-iterate of sinh(x) by using taylor series , we get a good approximation of the half-iterate of exp(x) for x large.

if this good approximation is analytic in say [e^e,e^e^e^e] we could use that interval and take logs or exp of it to compute the half-iterate for [-oo,+oo] up to a relatively high precision.

in fact , i assume , we can choose our precision if the good approximation is entire , by using bigger intervals and taking logs of it ( as somewhat done above ).

we might even consider the mittag-leffler expansion to avoid problems with non-analytic issues ( thus taking mittag-leffler expansion of the formal taylor series )

thats basicly the idea , but as said maybe sinh(x) isnt the best , on the other hand its probably the easiest.

furthermore some want - and me too actually - that half-iterates have non-negative (2+n)th derivatives and strictly positive zeroth , first and second derivatives.

that last restriction troubles me , especially when i try to get closer to exp(x) than sinh(x) by using function that have non-negative derivatives ... getting conditions on a particular n'th derivate isnt hard but the whole problem is more troublesome ( maybe someone knows a solution to this ? )

i didnt mention that all the above ofcourse is about functions going from R -> R , its obvious , but i just mention it to avoid potential confusion.

also every approximation of exp(x) should equal x at x = 0 only and be larger than id(x).

hope its clear.

maybe you considered this once too ?


greetings my fellow euh ... tetrationalists

regards

tommy1729

ps : showing that all derivatives are positive can sometimes be easy , but in general its hard , see for example " li's criterion " for the RH.
maybe its easy here and i missed a trivial thing ... ( im getting old ? :p )
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Messages In This Thread
using sinh(x) ? - by tommy1729 - 02/28/2010, 12:22 AM
RE: using sinh(x) ? - by tommy1729 - 03/06/2010, 12:35 PM
RE: using sinh(x) ? - by bo198214 - 03/07/2010, 11:19 AM
using 2* sinh(x) ! - by tommy1729 - 03/09/2010, 01:31 PM
RE: using 2* sinh(x) ! - by bo198214 - 03/10/2010, 11:38 AM
RE: using 2* sinh(x) ! - by tommy1729 - 03/11/2010, 12:50 AM
RE: using 2* sinh(x) ! - by tommy1729 - 03/14/2010, 01:07 AM
RE: using 2* sinh(x) ! - by sheldonison - 06/02/2010, 04:47 PM
RE: using 2* sinh(x) ! - by tommy1729 - 06/02/2010, 10:13 PM
RE: using 2* sinh(x) ! - by sheldonison - 06/02/2010, 10:56 PM
RE: using 2* sinh(x) ! - by tommy1729 - 06/07/2010, 12:34 AM
RE: using 2* sinh(x) ! - by sheldonison - 06/08/2010, 01:59 PM
RE: using 2* sinh(x) ! - by tommy1729 - 06/09/2010, 12:18 PM
RE: using 2* sinh(x) ! - by mike3 - 06/11/2010, 02:23 AM
RE: using 2* sinh(x) ! - by bo198214 - 06/12/2010, 04:36 AM
RE: using 2* sinh(x) ! - by mike3 - 06/12/2010, 11:14 AM
RE: using 2* sinh(x) ! - by tommy1729 - 06/21/2010, 09:14 PM
RE: using 2* sinh(x) ! - by sheldonison - 06/23/2010, 06:06 PM
RE: using 2* sinh(x) ! - by tommy1729 - 06/23/2010, 09:11 PM
RE: using 2* sinh(x) ! - by tommy1729 - 06/23/2010, 09:31 PM
RE: using 2* sinh(x) ! - by bo198214 - 06/27/2010, 09:16 AM
RE: using 2* sinh(x) ! - by sheldonison - 06/04/2010, 04:48 AM
RE: using sinh(x) ? - by tommy1729 - 04/24/2010, 11:47 PM
RE: using sinh(x) ? - by bo198214 - 05/31/2010, 05:52 AM
RE: using sinh(x) ? - by tommy1729 - 06/01/2010, 10:58 PM
RE: using sinh(x) ? - by bo198214 - 06/02/2010, 02:58 AM
RE: using sinh(x) ? - by tommy1729 - 06/22/2010, 11:18 PM
RE: using sinh(x) ? - by tommy1729 - 06/23/2010, 10:44 PM
RE: using sinh(x) ? - by tommy1729 - 06/24/2010, 12:29 PM
RE: using sinh(x) ? - by tommy1729 - 06/24/2010, 07:22 PM
RE: using sinh(x) ? - by bo198214 - 06/26/2010, 12:04 PM
RE: using sinh(x) ? - by tommy1729 - 06/26/2010, 09:19 PM
attracting fixed point of 2sinh - by sheldonison - 06/27/2010, 03:31 AM
RE: using sinh(x) ? - by tommy1729 - 07/18/2010, 10:41 PM
RE: using sinh(x) ? - by sheldonison - 07/19/2010, 09:37 PM
RE: using sinh(x) ? - by tommy1729 - 07/20/2010, 08:24 PM
RE: using sinh(x) ? - by tommy1729 - 07/19/2010, 10:20 PM
RE: using sinh(x) ? - by sheldonison - 07/20/2010, 03:13 PM
RE: using sinh(x) ? - by tommy1729 - 07/20/2010, 08:31 PM
RE: using sinh(x) ? - by sheldonison - 07/21/2010, 03:19 AM
RE: using sinh(x) ? - by tommy1729 - 07/21/2010, 10:34 PM
RE: using sinh(x) ? - by sheldonison - 07/22/2010, 12:23 PM
RE: using sinh(x) ? - by tommy1729 - 07/28/2010, 02:24 PM
RE: using sinh(x) ? - by tommy1729 - 07/28/2010, 02:53 PM
RE: using sinh(x) ? - by tommy1729 - 12/04/2010, 11:09 PM
RE: using sinh(x) ? - by tommy1729 - 12/08/2010, 02:31 PM
RE: using sinh(x) ? - by sheldonison - 01/17/2011, 07:43 PM
RE: using sinh(x) ? - by tommy1729 - 01/17/2011, 11:36 PM
RE: using sinh(x) ? - by tommy1729 - 07/02/2011, 10:32 PM
RE: using sinh(x) ? - by tommy1729 - 07/04/2011, 10:56 PM
RE: using sinh(x) ? - by sheldonison - 07/05/2011, 01:09 AM
RE: using sinh(x) ? - by tommy1729 - 07/05/2011, 12:06 PM
RE: using sinh(x) ? - by tommy1729 - 11/18/2012, 11:41 PM
RE: using sinh(x) ? - by sheldonison - 11/20/2012, 12:01 AM
RE: using sinh(x) ? - by tommy1729 - 11/20/2012, 10:53 PM
RE: using sinh(x) ? - by sheldonison - 11/20/2012, 11:01 PM
RE: using sinh(x) ? - by tommy1729 - 11/20/2012, 11:32 PM
RE: using sinh(x) ? - by sheldonison - 11/20/2012, 11:46 PM
RE: using sinh(x) ? - by tommy1729 - 11/28/2012, 06:59 PM
RE: using sinh(x) ? - by tommy1729 - 11/28/2012, 08:07 PM
RE: using sinh(x) ? - by tommy1729 - 12/17/2012, 04:34 PM
RE: using sinh(x) ? - by tommy1729 - 02/10/2015, 12:02 AM
RE: using sinh(x) ? - by tommy1729 - 04/22/2015, 12:27 PM
RE: using sinh(x) ? - by tommy1729 - 04/23/2015, 04:49 PM
RE: using sinh(x) ? - by tommy1729 - 10/26/2015, 12:56 AM
RE: using sinh(x) ? - by tommy1729 - 11/26/2015, 11:56 PM
RE: using sinh(x) ? - by sheldonison - 11/27/2015, 03:41 PM
RE: using sinh(x) ? - by tommy1729 - 11/28/2015, 01:21 PM
RE: using sinh(x) ? - by sheldonison - 11/29/2015, 03:03 PM
RE: using sinh(x) ? - by tommy1729 - 12/01/2015, 01:22 PM
RE: using sinh(x) ? - by sheldonison - 12/01/2015, 02:58 PM
RE: using sinh(x) ? - by tommy1729 - 12/02/2015, 12:04 AM
RE: using sinh(x) ? - by tommy1729 - 12/01/2015, 08:18 PM
RE: using sinh(x) ? - by tommy1729 - 12/02/2015, 12:28 AM
RE: using sinh(x) ? - by tommy1729 - 02/23/2016, 10:36 PM
RE: using sinh(x) ? - by tommy1729 - 02/24/2016, 01:27 PM
RE: using sinh(x) ? - by tommy1729 - 03/01/2016, 10:38 PM
RE: using sinh(x) ? - by tommy1729 - 03/05/2016, 01:27 PM
RE: using sinh(x) ? - by tommy1729 - 03/05/2016, 11:38 PM
RE: using sinh(x) ? - by sheldonison - 03/06/2016, 03:16 PM
RE: using sinh(x) ? - by tommy1729 - 03/06/2016, 10:54 PM
RE: using sinh(x) ? - by tommy1729 - 03/07/2016, 01:48 AM
RE: using sinh(x) ? - by tommy1729 - 03/07/2016, 10:46 AM
RE: using sinh(x) ? - by tommy1729 - 03/07/2016, 09:36 PM
RE: using sinh(x) ? - by tommy1729 - 03/08/2016, 12:26 AM
RE: using sinh(x) ? - by tommy1729 - 03/11/2016, 01:27 PM
RE: using sinh(x) ? - by tommy1729 - 03/12/2016, 01:12 PM
RE: using sinh(x) ? - by tommy1729 - 03/12/2016, 01:20 PM

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