#This file was created by <bruno> Sun Feb 16 14:19:06 1997
#LyX 0.10 (C) 1995 1996 Matthias Ettrich and the LyX Team
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\def\ll{\langle\!\langle}
\def\gg{\rangle\!\rangle}
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\layout LaTeX Title

The diagonal of a rational function
\layout Description

Theorem:
\layout Standard

Let 
\begin_inset Formula  \( M \)
\end_inset 

 be a torsion-free 
\begin_inset Formula  \( R \)
\end_inset 

-module, and 
\begin_inset Formula  \( d>0 \)
\end_inset 

.
 Let 
\begin_inset Formula 
\[
f=\sum _{n_{1},...,n_{d}}a_{n_{1},...,n_{d}}\, x_{1}^{n_{1}}\cdots x_{d}^{n_{d}}\in M[[x_{1},\ldots x_{d}]]\]

\end_inset 

be a rational function, i.
e.
 there are 
\begin_inset Formula  \( P\in M[x_{1},\ldots ,x_{d}] \)
\end_inset 

 and 
\begin_inset Formula  \( Q\in R[x_{1},\ldots ,x_{d}] \)
\end_inset 

 with 
\begin_inset Formula  \( Q(0,\ldots ,0)=1 \)
\end_inset 

 and 
\begin_inset Formula  \( Q\cdot f=P \)
\end_inset 

.
 Then the full diagonal of 
\begin_inset Formula  \( f \)
\end_inset 


\begin_inset Formula 
\[
g=\sum ^{\infty }_{n=0}a_{n,\ldots ,n}\, x_{1}^{n}\]

\end_inset 

is a D-finite element of 
\begin_inset Formula  \( M[[x_{1}]] \)
\end_inset 

, w.
r.
t.
 
\begin_inset Formula  \( R[x_{1}] \)
\end_inset 

 and 
\begin_inset Formula  \( \{\partial _{x_{1}}\} \)
\end_inset 

.

\layout Description

Proof:
\layout Standard

From the hypotheses, 
\begin_inset Formula  \( M[[x_{1},\ldots ,x_{d}]] \)
\end_inset 

 is a torsion-free differential module over 
\begin_inset Formula  \( R[x_{1},\ldots ,x_{d}] \)
\end_inset 

 w.
r.
t.
 the derivatives 
\begin_inset Formula  \( \{\partial _{x_{1}},\ldots ,\partial _{x_{d}}\} \)
\end_inset 

, and 
\begin_inset Formula  \( f \)
\end_inset 

 is a D-finite element of 
\begin_inset Formula  \( M[[x_{1},\ldots ,x_{d}]] \)
\end_inset 

 over 
\begin_inset Formula  \( R[x_{1},\ldots ,x_{d}] \)
\end_inset 

 w.
r.
t.
 
\begin_inset Formula  \( \{\partial _{x_{1}},\ldots ,\partial _{x_{d}}\} \)
\end_inset 

.
 Now apply the general diagonal theorem ([1], section 2.
18) 
\begin_inset Formula  \( d-1 \)
\end_inset 

 times.

\layout Description

Theorem:
\layout Standard

Let 
\begin_inset Formula  \( R \)
\end_inset 

 be an integral domain of characteristic 0 and 
\begin_inset Formula  \( M \)
\end_inset 

 simultaneously a torsion-free 
\begin_inset Formula  \( R \)
\end_inset 

-module and a commutative 
\begin_inset Formula  \( R \)
\end_inset 

-algebra without zero divisors.
 Let 
\begin_inset Formula 
\[
f=\sum _{m,n\geq 0}a_{m,n}x^{m}y^{n}\in M[[x,y]]\]

\end_inset 

 be a rational function.
 Then the diagonal of 
\begin_inset Formula  \( f \)
\end_inset 


\begin_inset Formula 
\[
g=\sum ^{\infty }_{n=0}a_{n,n}\, x^{n}\]

\end_inset 

 is algebraic over 
\begin_inset Formula  \( R[x] \)
\end_inset 

.

\layout Description

Motivation
\protected_separator 
of
\protected_separator 
proof:
\layout Standard

The usual proof ([2]) uses complex analysis and works only for 
\begin_inset Formula  \( R=M=C \)
\end_inset 

.
 The idea is to compute
\begin_inset Formula 
\[
g(x^{2})=\frac{1}{2\pi i}\oint _{|z|=1}f(xz,\frac{x}{z})\frac{dz}{z}\]

\end_inset 

This integral, whose integrand is a rational function in 
\begin_inset Formula  \( x \)
\end_inset 

 and 
\begin_inset Formula  \( z \)
\end_inset 

, is calculated using the residue theorem.
 Since 
\begin_inset Formula  \( f(x,y) \)
\end_inset 

 is continuous at 
\begin_inset Formula  \( (0,0) \)
\end_inset 

, there is a 
\begin_inset Formula  \( \delta >0 \)
\end_inset 

 such that 
\begin_inset Formula  \( f(x,y)\neq \infty  \)
\end_inset 

 for 
\begin_inset Formula  \( |x|<\delta  \)
\end_inset 

, 
\begin_inset Formula  \( |y|<\delta  \)
\end_inset 

.
 It follows that for all 
\begin_inset Formula  \( \varepsilon >0 \)
\end_inset 

 and 
\begin_inset Formula  \( |x|<\delta \varepsilon  \)
\end_inset 

 all the poles of 
\begin_inset Formula  \( f(xz,\frac{x}{z}) \)
\end_inset 

 are contained in 
\begin_inset Formula  \( \{z:|z|<\varepsilon \}\cup \{z:|z|>\frac{1}{\varepsilon }\} \)
\end_inset 

.
 Thus the poles of 
\begin_inset Formula  \( f(xz,\frac{x}{z}) \)
\end_inset 

, all algebraic functions of 
\begin_inset Formula  \( x \)
\end_inset 

 -- let's call them 
\begin_inset Formula  \( \zeta _{1}(x),\ldots \zeta _{s}(x) \)
\end_inset 

 --, can be divided up into those for which 
\begin_inset Formula  \( |\zeta _{i}(x)|=O(|x|) \)
\end_inset 

 as 
\begin_inset Formula  \( x\rightarrow 0 \)
\end_inset 

 and those for which 
\begin_inset Formula  \( \frac{1}{|\zeta _{i}(x)|}=O(|x|) \)
\end_inset 

 as 
\begin_inset Formula  \( x\rightarrow 0 \)
\end_inset 

.
 It follows from the residue theorem that for 
\begin_inset Formula  \( |x|<\delta  \)
\end_inset 


\begin_inset Formula 
\[
g(x^{2})=\sum _{\zeta =0\vee \zeta =O(|x|)}\Res _{z=\zeta }\, f(xz,\frac{x}{z})\]

\end_inset 

 This is algebraic over 
\begin_inset Formula  \( C(x) \)
\end_inset 

.
 Hence 
\begin_inset Formula  \( g(x) \)
\end_inset 

 is algebraic over 
\begin_inset Formula  \( C(x^{1/2}) \)
\end_inset 

, hence also algebraic over 
\begin_inset Formula  \( C(x) \)
\end_inset 

.

\layout Description

Proof:
\layout Standard

Let 
\begin_inset Formula 
\[
h(x,z):=f(xz,\frac{x}{z})=\sum ^{\infty }_{m,n=0}a_{m,n}x^{m+n}z^{m-n}\in M[[xz,xz^{-1}]]\]

\end_inset 

Then 
\begin_inset Formula  \( g(x^{2}) \)
\end_inset 

 is the coefficient of 
\begin_inset Formula  \( z^{0} \)
\end_inset 

 in 
\begin_inset Formula  \( h(x,z) \)
\end_inset 

.
 Let 
\begin_inset Formula  \( N(x,z):=z^{d}Q(xz,\frac{x}{z}) \)
\end_inset 

 (with 
\begin_inset Formula  \( d:=\max (\deg _{y}P,\deg _{y}Q) \)
\end_inset 

) be 
\begin_inset Quotes eld
\end_inset 

the denominator
\begin_inset Quotes erd
\end_inset 

 of 
\begin_inset Formula  \( h(x,z) \)
\end_inset 

.
 We have 
\begin_inset Formula  \( N(x,z)\in R[x,z] \)
\end_inset 

 and 
\begin_inset Formula  \( N\neq 0 \)
\end_inset 

 (because 
\begin_inset Formula  \( N(0,z)=z^{d} \)
\end_inset 

).
 Let 
\begin_inset Formula  \( K \)
\end_inset 

 be the quotient field of 
\begin_inset Formula  \( R \)
\end_inset 

.
 Thus 
\begin_inset Formula  \( N(x,z)\in K[x][z]\setminus \{0\} \)
\end_inset 

.

\layout Standard

It is well-known (see [3], p.
64, or [4], chap.
 IV, §2, prop.
 8, or [5], chap.
 III, §1) that the splitting field of 
\begin_inset Formula  \( N(x,z) \)
\end_inset 

 over 
\begin_inset Formula  \( K(x) \)
\end_inset 

 can be embedded into a field 
\begin_inset Formula  \( L((x^{1/r})) \)
\end_inset 

, where 
\begin_inset Formula  \( r \)
\end_inset 

 is a positive integer and 
\begin_inset Formula  \( L \)
\end_inset 

 is a finite-algebraic extension field of 
\begin_inset Formula  \( K \)
\end_inset 

, i.
e.
 a simple algebraic extension 
\begin_inset Formula  \( L=K(\alpha )=K\alpha ^{0}+\cdots +K\alpha ^{u-1} \)
\end_inset 

.
 
\layout Standard


\begin_inset Formula  \( \widetilde{M}:=(R\setminus \{0\})^{-1}\cdot M \)
\end_inset 

 is a 
\begin_inset Formula  \( K \)
\end_inset 

-vector space and a commutative 
\begin_inset Formula  \( K \)
\end_inset 

-algebra without zero divisors.
 
\begin_inset Formula  \( \widehat{M}:=\widetilde{M}\alpha ^{0}+\cdots +\widetilde{M}\alpha ^{u-1} \)
\end_inset 

 is an 
\begin_inset Formula  \( L \)
\end_inset 

-vector space and a commutative 
\begin_inset Formula  \( L \)
\end_inset 

-algebra without zero divisors.
 
\layout Standard


\begin_inset Formula 
\begin{eqnarray*}
\widehat{M}\ll x,z\gg  & := & \widehat{M}[[x^{1/r}\cdot z,x^{1/r}\cdot z^{-1},x^{1/r}]][x^{-1/r}]\\
 & = & \left\{ \sum _{m,n}c_{m,n}x^{m/r}z^{n}:c_{m,n}\neq 0\Rightarrow |n|\leq m+O(1)\right\} 
\end{eqnarray*}

\end_inset 

is an 
\begin_inset Formula  \( L \)
\end_inset 

-algebra which contains 
\begin_inset Formula  \( \widehat{M}((x^{1/r})) \)
\end_inset 

.

\layout Standard

Since 
\begin_inset Formula  \( N(x,z) \)
\end_inset 

 splits into linear factors in 
\begin_inset Formula  \( L((x^{1/r}))[z] \)
\end_inset 

, 
\begin_inset Formula  \( N(x,z)=l\prod ^{s}_{i=1}(z-\zeta _{i}(x))^{k_{i}} \)
\end_inset 

, there exists a partial fraction decomposition of 
\begin_inset Formula  \( h(x,z)=\frac{P(xz,\frac{x}{z})}{Q(xz,\frac{x}{z})}=\frac{z^{d}P(xz,\frac{x}{z})}{N(x,z)} \)
\end_inset 

 in 
\begin_inset Formula  \( \widehat{M}\ll x,z\gg  \)
\end_inset 

:
\layout Standard


\begin_inset Formula 
\[
h(x,z)=\sum ^{l}_{j=0}P_{j}(x)z^{j}+\sum ^{s}_{i=1}\sum ^{k_{i}}_{k=1}\frac{P_{i,k}(x)}{(z-\zeta _{i}(x))^{k}}\]

\end_inset 

with 
\begin_inset Formula  \( P_{j}(x),P_{i,k}(x)\in \widehat{M}((x^{1/r})) \)
\end_inset 

.

\layout Standard

Recall that we are looking for the coefficient of 
\begin_inset Formula  \( z^{0} \)
\end_inset 

 in 
\begin_inset Formula  \( h(x,z) \)
\end_inset 

.
 We compute it separately for each summand.

\layout Standard

If 
\begin_inset Formula  \( \zeta _{i}(x)=ax^{m/r}+... \)
\end_inset 

 with 
\begin_inset Formula  \( a\in L\setminus \{0\} \)
\end_inset 

, 
\begin_inset Formula  \( m>0 \)
\end_inset 

, or 
\begin_inset Formula  \( \zeta _{i}(x)=0 \)
\end_inset 

, we have
\layout Standard


\begin_inset Formula 
\begin{eqnarray*}
\frac{1}{(z-\zeta _{i}(x))^{k}} & = & \frac{1}{z^{k}}\cdot \frac{1}{\left( 1-\frac{\zeta _{i}(x)}{z}\right) ^{k}}\\
 & = & \frac{1}{z^{k}}\cdot \sum ^{\infty }_{j=0}{k-1+j\choose k-1}\left( \frac{\zeta _{i}(x)}{z}\right) ^{j}\\
 & = & \sum ^{\infty }_{j=0}{k-1+j\choose k-1}\frac{\zeta _{i}(x)^{j}}{z^{k+j}}
\end{eqnarray*}

\end_inset 

hence the coefficient of 
\begin_inset Formula  \( z^{0} \)
\end_inset 

 in 
\begin_inset Formula  \( \frac{P_{i,k}(x)}{(z-\zeta _{i}(x))^{k}} \)
\end_inset 

 is 
\begin_inset Formula  \( 0 \)
\end_inset 

.

\layout Standard
\cursor 59 
If 
\begin_inset Formula  \( \zeta _{i}(x)=ax^{m/r}+... \)
\end_inset 

 with 
\begin_inset Formula  \( a\in L\setminus \{0\} \)
\end_inset 

, 
\begin_inset Formula  \( m<0 \)
\end_inset 

, we have
\begin_inset Formula 
\[
\frac{1}{(z-\zeta _{i}(x))^{k}}=\frac{1}{(-\zeta _{i}(x))^{k}}\cdot \frac{1}{\left( 1-\frac{z}{\zeta _{i}(x)}\right) ^{k}}=\frac{1}{(-\zeta _{i}(x))^{k}}\cdot \sum _{j=0}^{\infty }{k-1+j\choose k-1}\left( \frac{z}{\zeta _{i}(x)}\right) ^{j}\]

\end_inset 

hence the coefficient of 
\begin_inset Formula  \( z^{0} \)
\end_inset 

 in 
\begin_inset Formula  \( \frac{P_{i,k}(x)}{(z-\zeta _{i}(x))^{k}} \)
\end_inset 

 is 
\begin_inset Formula  \( \frac{P_{i,k}(x)}{(-\zeta _{i}(x))^{k}} \)
\end_inset 

.

\layout Standard

The case 
\begin_inset Formula  \( \zeta _{i}(x)=ax^{m/r}+... \)
\end_inset 

 with 
\begin_inset Formula  \( a\in L\setminus \{0\} \)
\end_inset 

, 
\begin_inset Formula  \( m=0 \)
\end_inset 

, cannot occur, because it would imply 
\begin_inset Formula  \( 0=N(0,\zeta _{i}(0))=N(0,a)=a^{d}. \)
\end_inset 


\layout Standard

Altogether we have
\begin_inset Formula 
\[
g(x^{2})=[z^{0}]h(x,z)=P_{0}(x)+\sum _{\frac{1}{\zeta _{i}(x)}=o(x)}\sum ^{k_{i}}_{k=1}\frac{P_{i,k}(x)}{(-\zeta _{i}(x))^{k}}\in \widehat{M}((x^{1/r}))\]

\end_inset 


\layout Standard

Since all 
\begin_inset Formula  \( \zeta _{i}(x) \)
\end_inset 

(in 
\begin_inset Formula  \( L((x^{1/r})) \)
\end_inset 

) and all 
\begin_inset Formula  \( P_{j}(x),P_{i,k}(x) \)
\end_inset 

 (in 
\begin_inset Formula  \( \widehat{M}((x^{1/r})) \)
\end_inset 

) are algebraic over 
\begin_inset Formula  \( K(x) \)
\end_inset 

, the same holds also for 
\begin_inset Formula  \( g(x^{2}) \)
\end_inset 

.
 Hence 
\begin_inset Formula  \( g(x) \)
\end_inset 

 is algebraic over 
\begin_inset Formula  \( K(x^{1/2}) \)
\end_inset 

, hence also over 
\begin_inset Formula  \( K(x) \)
\end_inset 

.
 After clearing denominators, we finally conclude that 
\begin_inset Formula  \( g(x) \)
\end_inset 

 is algebraic over 
\begin_inset Formula  \( R[x] \)
\end_inset 

.

\layout Bibliography

[1] Bruno Haible: D-finite power series in several variables.
 
\shape italic 
Diploma thesis, University of Karlsruhe, June 1989.

\shape default 
 Sections 2.
18 and 2.
20.

\layout Bibliography

[2] M.
 L.
 J.
 Hautus, D.
 A.
 Klarner: The diagonal of a double power series.
 
\shape italic 
Duke Math.
 J.

\shape default 
 
\series bold 
38
\series default 
 (1971), 229-235.

\layout Bibliography

[3] C.
 Chevalley: Introduction to the theory of algebraic functions of one variable.
 
\shape italic 
Mathematical Surveys VI.
 American Mathematical Society.

\layout Bibliography

[4] Jean-Pierre Serre: Corps locaux.
 
\shape italic 
Hermann.
 Paris 
\shape default 
1968.

\layout Bibliography

[5] Martin Eichler: Introduction to the theory of algebraic numbers and
 functions.

\shape italic 
 Academic Press.
 New York, London 
\shape default 
1966.