(05/16/2014, 07:27 AM)sheldonison Wrote: The next graph is f(z), the asymptotic half iterate itself, using the same grid coordinates. You can see zeros for f on the real axis, as black dots, at -0.71, -4.26, and -15.21. The pattern goes on forever, as f grows at the negative real axis, oscillating between positive and negative.
From seeing the plot I was reminded of 3 ideas I had.
Basicly I find the interpretation in terms of polar cordinates often more intresting.
This leads to 3 ideas and a remark.
remark : you say " oscillating between positive and negative ".
Now I know that if a closed jordan curve path has all arguments n times then we have n zero's within that closed path.
So that suggests that all real roots have multiplicity 1 and you are in the possesion of a proof ?
Also you have claimed that there are no zero's off the real line ?
Does that have a proof ?
I think it should be provable for Re(z) > 0.
As for these 3 ideas , I consider f(z) somewhat as between z and exp(z) but also as in between a line and a circle.
Let me explain : if we consider id(z) = z then abs id(z) has contour circles of absolute value |z|.
exp(z) has vertical contour lines of absolute value.
The idea is that for Re(z) >> 0 , f(z) slightly bends those circles towards the lines and never crosses the lines by doing so.
Also the lines never intersect , just as they dont for id(z) and exp(z).
So we have for a >> 0 , |f(a)| =< |f(a+bi)|.
That also implies there are no zero's for f when a >> 0 !!
So far the absolute value and the zero's of f.
What else seems logical ?
also for a >> 0 ;
idea 2 : While b increases arg(f(a+bi)) takes on all values infinitely often.
The number of times arg(f(a+bi)) takes on a particular angle (for fixed a) is of the form O ( x0 + x1 b + x2 f(x3 b) + x4 exp(x5 b) ) where the x_i are constants.
This idea also comes from considering the bending of the circles to the lines and from the periodicity of exp. And also from the winding , because f has an infinite amount of zero's in the left plane.
In particular the integral f ' (z)/f(z) over the path from - oo i to + oo i is intresting.
Unfortunately this is not a zeta function nor general dirichlet series ?
Speaking of dirichlet series :
idea 3 : from the previous ideas it seems f(z) cannot be of the form
ln(a0 + a1 (c1)^z + a2 (c2)^z + ...)
for Re(z) >> 0 and all a_i , ln(c_i) > 0.
If however the ideas about absolute value are wrong it is possible !
Those are the ideas I had.
It seems some theorems from complex analysis and root finding algoritms might be needed.
Although as Always the functions considered are nontrivial , nonelementary etc. but general theorems might work.
Now for the truncated Taylor series ( polynomials ) we get that abs has a simple structure for large z ; we approach a circle.
Same for the truncated hadamard , since that is also a polynomial.
So , should we study polynomials and or small z together with the absolute value ?
Or is that the wrong way ?
Another issue is that polynomials have zero's while we might want to consider f where it is not 0. that might affect the quality of the approximation of the abs.
Final remark :
Isnt there some theorem that says all zero's are have negative real part for polynomials with positive coefficients or such ?
I seem to recall something like that.
regards
tommy1729