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How to design algebraic function with particular ramified covering?

by jackmell
Tags: algebraic, covering, design, function, ramified
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Nov13-12, 01:08 PM
P: 1,666

I wish to study (non-trivial) algebraic functions which have varied cycles at a few places other than the origin. It's not hard to search for a particular covering at the origin. For example, the function

[tex]f(z,w)=(8 z^2)\text{}+(-10 z-4 z^2-6 z^3)w+(-10 z)w^2+(-3 z^2-6 z^3)w^3+(-9 z^2+4 z^4)w^4+(6-5 z+z^3)w^5[/tex]

has a 1-cycle and 4-cycle at the origin. I just randomly searched for one. However, it seems it's not so easy to find one with higher cycles than a 2-cycle outside the origin or maybe I'm missing something. I'd like to find some with more than one such point. I can certainly make a translation say:


and now the function [itex]g(z,w)[/itex] will have a 1-cycle and a 4-cycle at the point z=1. But that's as far as I can go.

How would I design one with say two such cycle types say at the points -1 and 1?

I don't think just making the substitution g(z-1,w) would do it.

So is there a way to design an algebraic function [itex]f(z,w)[/itex] so that it will have a particular ramified covering at several points outside the origin or is there an easy way to search for ones other than just randomly?

Edit: Just thought of a more cioncise way of asking my question:

How can I find a non-trivial algebraic function of degree at least 5 in [itex]p_n(z)[/itex] and w so that it has a non-trivial ramified covering above several of it's non-zero singular points and by non-trivial, I mean coverings which are more ramified than a set of single-cover branches with one 2-cycle branch?

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Nov21-12, 09:17 AM
P: 1,666
Hey guys. Turns out to be a trival matter. Let's take a simple example:

[tex]f(z,w)=p_0+p_1 w+p_2 w^2+p_3 w^3[/tex]

and suppose we wish to have it fully-ramify at the points [itex]z=1,-1[/itex]. A sufficient condition for full ramification of the translated function [itex]f(z\pm 1,w)[/itex], is for the lower Newton-leg of it's first polygon to have a slope of [itex]1/3[/itex]. Thus, one possibility is:


where each [itex]q_i[/itex] is a suitable polynomial in [itex]z[/itex]. Take [itex]p_0(z)=a+bz+cz^2[/itex]. Then one solution is: [itex]a+b(z\pm 1)+c(z\pm 1)^2=\alpha z+\beta z^2[/itex] or [itex]a\pm b+c=0[/itex]. Let's take b=0, a=1 and c=-1 so that for the first term, [itex]p_0(z)=1-z^2[/itex]. We can do the same for the others and arrive at a suitable function:

[tex]f(z,w)=(1-z^2)+(1-z^2)w+(1-z^2)w^2+z w^3[/tex]

with the desired ramification geometry at the points z=-1 and 1.

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