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“ except for some unknown constants, called the central connection coef-
¬cients. The computation of these coe¬cients is what this problem is all
about, and we shall show in which sense this can be done.

12.1 Matrix Methods and Multisummability
We recall from Chapter 6 the following notion: Given an in¬nite matrix
A = (amn ), with amn ∈ C for m, n ∈ N0 , we call a formal power series
ˆ fn z n ∈ E [[z]] A-summable in a direction d ∈ R, if there exists a
f (z) =
sectorial region G = G(d, ±) of opening ± > 0, such that the following two
conditions hold:

amn fn z n converge in discs that all contain
1) The series fm (z) = n=0
G, for every m ∈ N0 .

2) The limit f (z) = limm’∞ fm (z) exists uniformly on every closed
subsector of G.

The so de¬ned function f , which is holomorphic on G, will be called the
A-sum of f on G. The set of all formal power series that are A-summable in
direction d will be denoted by E {z}A,d , and we write SA,d f for the A-sum
of f .
In this context, it is natural to call a matrix A weakly p-regular (with p
being short for power series), if every convergent series is A-summable in
every direction d to the correct sum. A necessary and su¬cient condition
for this to hold is given in the exercises. Observe, however, that weak p-
regularity is not the same as power series regularity: Let f (z) converge,
say, for |z| < r. Then for a weakly p-regular matrix we have that to every d
there exists G(d, ±) so that the fm converge to f = S f on G(d, ±), and the
union of these regions may be a proper subset of the disc of convergence;
see the exercises below that this can occur. The de¬nition of power series
regular matrices, however, requires this union to be the full disc.
The following problem has been solved by Beck [29, 50] for the case of
E = C , but the proofs carry over to a general Banach space: Characterize
those matrices A for which the following comparison condition (C) holds:
12.1 Matrix Methods and Multisummability 185

(C) For every Banach space E , every k > 1/2 and every d ∈ R, we
ˆ ˆ
have E {z}k,d ‚ E {z}A,d , and (Sk,d f )(z) = (SA,d f )(z) for every
f ∈ E {z}k,d and all z where both sides are de¬ned.

Because of E {z} ‚ E {z}k,d , we see that weak p-regularity is a necessary
condition for (C), while power series regularity is not. For that reason, we
shall from now on only consider matrices A that are weakly p-regular. Also,
note that (Sk,d f )(z) always is holomorphic on a region of opening larger
than π/k and bisecting direction d, while (SA,d f )(z) is, in general, only
de¬ned close to the bisecting ray.
To give necessary and su¬cient conditions for (C), we introduce the

following terminology: For m ≥ 0, we de¬ne km (z) = n=0 amn z n . Then
we say that A = (amn ) satis¬es the regularity condition (R), if the following

(R) The functions km (z), m ∈ N0 , are all entire, and converge compactly
to (1 ’ z)’1 , for m ’ ∞ and every z in the sector S(π, 2π).

Moreover, we say that A = (amn ) satis¬es the order condition (O), if the
km (z) are all entire functions of exponential order ¤ 1/2. Finally, we say
that A = (amn ) satis¬es the growth condition (G), if the following holds:

(G) The functions are all entire, and for every k > 1/2 and every σ with
1/k < σ < 2 there exist c, K > 0 such that |km (z)| ¤ c eK |z| , for
every z ∈ S(π, (2 ’ σ)π) and every m ∈ N0 .

Observe that the growth condition becomes meaningless for k ¤ 1/2, since
then the interval for σ is empty. This is why we here restrict ourselves to
k > 1/2. Also note that the constants c, K in the estimate are independent
of m.
Let a weakly p-regular matrix A be given and assume that (C) holds.
For every k > 1/2 and d with 0 < d < 2π, the formal series fk (z) =

0 “(1 + n/k) z is in C {z}k,d , and hence must be A-summable in every

such direction d. This implies that the power series 0 anm “(1 + n/k) z n
must have a positive radius of convergence, for every such k and every
m ≥ 0. From this we conclude the existence of c, K > 0, depending on m
and k but independent of n, so that |amn | ¤ c K n /“(1 + n/k) for every
n ≥ 0. Thus, the order condition (O) follows. Moreover, by de¬nition of
A-summability we have the existence of µ, r > 0, depending on d and k, so

that the functions fm,k (z) = 0 anm “(1 + n/k) z n , for m ’ ∞, converge
uniformly on S(d, µ), for every d as above. A compactness argument then
shows uniform convergence, hence boundedness, of the fm,k on arbitrary
closed subsectors S of S(π, 2π). Since km = Bk fm,k , we can use an estimate
as in the proof of Theorem 24 (p. 82) to show (G). Finally, interchanging
Borel transform and limit, we can conclude that the kernel functions km (z)
186 12. Other Related Questions

converge to (1’z)’1 , and convergence is locally uniform, in S(π, (2’1/k)π).
Hence, using that k can be taken arbitrarily large we see that (R) holds.
So in shorthand notation, we have shown that (C) implies (O), (R), and
(G). The converse also holds, as we now show:
Theorem 59 Let a weakly p-regular in¬nite matrix A be given. Then (C)
holds if and only if (R), (O), and (G) are satis¬ed.

Proof: One direction of the proof has already been given, so we now
assume that (R), (O), and (G) are satis¬ed. For d ∈ R and k > 1/2,
consider a series f ∈ E {z}k,d . As shown in the proof of Theorem 41 (p. 120),
we can decompose f into a convergent series plus ¬nitely many others
which are moment series; so without loss of generality we can restrict f
to have coe¬cients of the form (7.1) (p. 116), with ψ ∈ A1/k,0 (G, E ), and
d + π/(2k) < arg a < d + (2 ’ 1/(2k))π; compare Remark 9 (p. 117) to see
that then f ∈ E {z}k,d . In this case, we have
∞ a
1 du
fm (z) = amn fn z = ψ(u) km (z/u) ,
2πi u

the interchange of summation and integration being justi¬ed because of
(O). For arg u su¬ciently close to d, we can then use (G) to justify inter-
changing integration and limit as m ’ ∞ to obtain with help of (R):
1 ψ(u)
lim fm (z) = du,
m’∞ 0

and the right-hand side is equal to Sk,d f . 2
Let now any multisummability type k = (k1 , . . . , kq ) be given, assuming
k1 > . . . > kq > 1/2. Then the parameters κj , given by κ1 = k1 , 1/κj =
1/kj ’ 1/kj’1 , 2 ¤ j ¤ q, automatically are larger than 1/2, so that the
Main Decomposition Theorem (p. 164) applies. This shows that the above
theorem immediately generalizes to multisummable series “ however, we
have to restrict to multidirections d = (d1 , . . . , dq ) with d1 = . . . = dq ,
because otherwise there may be no common sector on which the A-sum of
a multisummable series can be de¬ned.
Not very many matrix methods seem to satisfy the conditions (O), (R),
(G): Jurkat [144] studied the matrices J± = (jmn (±)) with jmn (±) =
exp[’δm »(±n)], where δm may be any positive sequence tending to 0 as
m ’ ∞, ± is a positive real parameter, and

»(u) = u log(u + 3) log log(u + 3).

This method had already been introduced by Hardy [113] for summation
of special power series with rapidly growing coe¬cients. It is a variant
12.2 The Method of Reduction of Rank 187

of what is called Lindel¨f ™s methods, useful for computing holomorphic
continuation. Jurkat showed that his method satis¬es all three conditions,
so that (C) follows. So far, this is the only method known to have this
property. On the other hand, Braun in [32] showed that no power series
method can have property (C).

Exercises: In the following exercises, consider a ¬xed matrix A = (amn )
that may not be weakly p-regular, and de¬ne km (z) as above, assuming
convergence for |z| < r, with r > 0 independent of m.

0 amn fn z , assume convergence for |z| < ρ, with
1. For fm (z) =
0 < ρ, independent of m. Derive the integral representation fm (z) =
(1/2πi) |u|=ρ f (u) km (z/u) (du/u), for ρ < ρ and |z| < ρ r.
˜ ˜

2. Show that A is weakly p-regular if and only if some r with 0 < r ¤ 1
exists, for which km (z) converge locally uniformly to (1 ’ z)’1 on
D(0, r). Conclude that power series regularity is equivalent to the
same with r = 1.

3. For a ∈ C with |a| = 1, let amn = e’am j=n (am)j /j!, m, n ≥ 0.
Use the previous exercise to conclude that this A is weakly p-regular
if and only if a has positive real part, and power series regular if and
only if a = 1.

12.2 The Method of Reduction of Rank
In this section we show that in some sense a system (3.1) (p. 37) of
Poincar´ rank r ≥ 2 is equivalent to one of rank r = 1, which will be
called the rank-reduced system. The process of rank-reduction has been
used by Poincar´ [222] and Birkho¬ [53] in representing certain solutions
of systems of higher rank as Laplace integrals. Also see Turrittin [270],
Lutz [174], Balser, Jurkat, and Lutz [38], and Sch¨fke and Volkmer [242].
Let a system (3.1), with r ≥ 2, be given, let µr = exp[2πi/r]. With 0ν ,
resp. Iν denoting the zero, resp. identity, matrix of dimension ν, de¬ne the
rν — rν matrices D = r’1 diag [0ν , Iν , 2 Iν , . . . , (r ’ 1) Iν ], and
® 
0ν 0ν . . . 0ν Iν
 Iν 0ν . . . 0ν 0ν 
 
U = . . .
. .
°. .»
. .
. . . .
0ν 0ν ... Iν 0ν

For an arbitrary fundamental solution X(z) of (3.1), set

Y (z) = z ’D U ’1 diag [X(z 1/r ), X(µr z 1/r ), . . . , X(µr’1 z 1/r )]U z D .
188 12. Other Related Questions

With some patience, one can then verify that Y (z) is a fundamental solution

of a system z y = B(z) y, B(z) = n=0 Bn z ’n , with coe¬cients of the
following form:
® 
Arn Arn+1 . . . Arn+r’2 Arn+r’1
 
Arn’1 Arn . . . Arn+r’3 Arn+r’2
 
n ≥ 0,
Bn =  ,
. . . .
° »
. . . .
. . . .
Arn’r+1 Arn’r+2 ... Arn’1 Arn

where A’r+1 = . . . = A’1 = 0, and the others are as in (3.1). In particular,
the new system has Poincar´ rank one. It is named the rank-reduced system
corresponding to (3.1). Note that the expansion of B(z) converges for |z| >
ρ1/r , with ρ > 0 as in (3.1). For many more formulas relating (3.1) and its
rank-reduced system, see [38].

12.3 The Riemann-Hilbert Problem
Let n distinct complex numbers aj be given, which for notational conve-
nience are assumed to be nonzero. Consider linear systems of ODE of the
form x = A(z) x, for z ∈ G = C \ {a1 , . . . , an }, and A(z) = j=1 (z ’
aj )’1 Aj , for given matrices Aj ∈ C ν—ν . Obviously, this is a system with
singular points at a1 , . . . , an and in¬nity, which all are singularities of ¬rst
kind; compare the discussion at the end of Section 1.6 on how to deter-
mine the type of singularity at in¬nity. Every system satisfying the above
requirements will be named a Fuchsian system.
Let such a Fuchsian system be given. Consider paths γj in G, originating
from the origin and going to points “near” aj in one way or another, then
encircling aj in the positive sense along a circle of small radius, and re-
tracing themselves back to the origin. According to results from Chapter 1,
there is a unique fundamental solution X(z) of our Fuchsian system sat-
isfying X(0) = I, holomorphic in some disc about the origin. We perform
its holomorphic continuation along the path γj , ending with a fundamental
solution Xγj (z), which will in general be di¬erent from X(z). In any case,
there exists an invertible constant matrix Cγj so that Xγj (z) = X(z) Cγj .


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