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k± (t, u) = E(wt) e(wu) ,
2πi w

with corresponding paths β± beginning at the point w = 1/r and going
o¬ to in¬nity. The integrals remain the same when changing the paths of
integration, provided that wu, for w on the path, stays within the sector
S+ where the kernel e(wu) is de¬ned, and wt, at least for large values of t,
5.8 Convolution of Kernels 93

remains in S’ where then E(wt) is bounded. By adding a corresponding
integral from 0 to 1/r, we can with help of Cauchy™s theorem also integrate
along the ray arg w = “(π + µ)/(2k). Doing so, and using (5.15), we obtain
’1 dw 1 u
k± (t, u) + E(wt) e(wu) = . (5.19)
2πi t ’ u
2πi w

This shows k+ (t, u) = k’ (t, u), and k± (t, u) is analytic for u ∈ S+ and
arbitrary t, except for t = u. Hence we obtain

(T ’ g)(t) = f+ (t) ’ f’ (t) = f (u) k± (t, u) ,

where the path γ is from in¬nity to the origin along the ray arg u = ’δ,
and back to in¬nity along arg u = δ. For |u| > |t|, we can deform both
rays to be along the real axis, and then these parts of the integral cancel.
Therefore, we ¬nd that γ may as well be replaced by a closed Jordan curve
encircling t and having negative orientation. This, together with (5.19) and
Cauchy™s formula, then implies T ’ g = f . 2

Exercises: In the following exercises, let e(z) be a kernel function of
order k > 0, and consider the corresponding integral operators T and T ’ .

1. Let S be a sector of in¬nite radius, and let A denote a subspace of
A(k) (S, E ) that is an asymptotic space, in the sense de¬ned on p. 75.
Show that then the image of A under the operator T is again an
asymptotic space.

2. Let G be a sectorial region of opening more than π/k, and let A de-
note a subspace of H(G, E ) which is an asymptotic space. Show that
then the image of A under the operator T ’ is again an asymptotic

5.8 Convolution of Kernels
In this section, we consider two pairs of operators Tj , Tj’ of order kj with
corresponding moment functions mj (u), 1 ¤ j ¤ 2. We will try to ¬nd a
+ +
third pair T, T ’ of operators, so that T coincides with T1 —¦ T2 , resp. T =
+ ’
T2 —¦ T1 , at least when applied to the geometric series. Consequently, the
corresponding m(u) equals either the product m2 (u) m1 (u) or the quotient
m2 (u)/m1 (u). In the ¬rst case the new operators clearly will have to have
order k = (1/k1 + 1/k2 )’1 , because it follows from Stirling™s formula that
m(n) is of order “(1 + n/k). In the second case, their order will be k =
94 5. Integral Operators

(1/k2 ’ 1/k1 )’1 , hence we here have to assume k1 > k2 . Because of T ’
being the inverse of T , at least for suitable function spaces, we only have
to ¬nd T , resp. its kernel function e(z).
The following lemma shows how one can recover e(z) from its moment
sequence m(n). However, observe that we shall not be concerned with the
harder question of how to characterize such m(n) to which a kernel e(z)

Lemma 7 Let a kernel function e(z) of order k with corresponding op-
erator T be given. For f (u) = (1 ’ u)’1 , let g = T f . Then g(z) is
holomorphic in for ’π/(2k) < arg z < (2 + 1/(2k))π, is asymptotic to

of Gevrey order s = 1/k there, and g(z) ’ 0 as
g (z) =
ˆ 0 m(n) z
z ’ ∞. Moreover,

g(z) ’ g(ze2πi ) = 2πi e(1/z), | arg z| < π/(2k). (5.20)

Proof: Holomorphy of the function g and its behavior at the origin follow
from Theorem 27 (p. 91), the behavior at in¬nity can be read o¬ from the
integral representation. To show (5.20), represent g(z) resp. g(ze2πi ) by
integrals (5.13) with „ = µ resp. „ = 2π ’ µ, for (small) µ > 0, and use the
Residue Theorem.

Remark 7: Observe that, according to Watson™s Lemma (p. 75), there is
only one g with g(z) ∼1/k m(n) z n in a sector of opening so large as in
the above lemma. So we indeed have shown that the kernel e(z) is uniquely
determined by (5.20).

Theorem 31 Let two kernel functions ej (z) of orders kj , with correspond-
ing moment functions mj (u) and operators Tj , 1 ¤ j ¤ 2, be given. Then
there is a unique kernel function e(z) of order k = (1/k1 +1/k2 )’1 with cor-
responding moment function m(u) = m1 (u)m2 (u). For the corresponding
integral operator T and f (u) = (1 ’ u)’1 we then have T f = (T1 —¦ T2 ) f =
(T2 —¦ T1 ) f . In particular, the function e(1/z) is given by applying T1 to the
function e2 (1/u).

Proof: Uniqueness follows from Lemma 7, resp. Remark 7. For existence,
let g(z) = (T1 —¦ T2 )f , f (u) = (1 ’ u)’1 , and de¬ne e(z) by (5.20). Inter-
changing the order of integration in (T1 —¦ T2 )f , show that e(1/z) is given by
applying T1 to e2 (1/u). Then, check that e(z) has the necessary properties
for a kernel function of this order as listed on p. 89; in particular, note that

the function Ep (z) can be obtained by applying T2 to the corresponding
function E1,p (z).
As the ¬nal result in this context, we now ¬nd the kernel corresponding
to the quotient of the moment functions:
5.8 Convolution of Kernels 95

Theorem 32 Given two kernel functions ej (z) of order kj with corre-
sponding moment functions mj (u), 1 ¤ j ¤ 2, assume k1 > k2 . Then
there exists a unique kernel function e(z) of order k = (1/k2 ’ 1/k1 )’1
corresponding to the moment function m(u) = m2 (u)/m1 (u). In particu-

lar, the function e(1/u) is given by an application of T1 to the function
e2 (1/z).

Proof: As above, uniqueness follows from Lemma 7, resp. Remark 7. For

existence, let f (u) = (1 ’ u)’1 and de¬ne g2 = T2 f . Then g = T1 g2 is
holomorphic in ’π/(2k) < arg w < (2 + 1/(2k))π, and we de¬ne e(z) by
(5.20). Interchanging the order of integration, one obtains that e(1/u) is

given by an application of T1 to the function e2 (1/z). To complete the
proof, one can verify the necessary properties for e(z), using Theorems 27
and 28.
As an application of the last theorem, we mention that for k1 = ± > 1,
e1 (z) = ± z ± exp[’z ± ], and k2 = 1, e2 (z) = z ez , the function C± (u) =
u’1 e(u) equals the kernel of Ecalle™s acceleration operator, which will be
studied in detail in Chapter 11. More kernels are constructed in the follow-
ing exercises.

1. Using the above results, verify existence of kernel functions corre-
sponding to moment functions of the form

“(1 + s1 u) · . . . · “(1 + sν u)
m(u) = , (5.21)
“(1 + σ1 u) · . . . · “(1 + σµ u)
ν µ

with positive parameters sj , σj satisfying j=1 sj σj > 0,
and ¬nd the order of the kernels.

2. Using the above results, verify existence of kernel functions of order
one corresponding to moment functions of the form

“(β1 + u) · . . . · “(βµ + u) “(1 + u)
µ ≥ 1,
m(u) = , (5.22)
“(±1 + u) · . . . · “(±µ + u)

with positive parameters ±j , βj . Relate E(z) to the generalized con-
¬‚uent hypergeometric function (p. 23).

3. Let f (z) = 0 m(n) z n , with m(n) as in (5.21), resp. (5.22), and let
k be the order of the corresponding kernel function. Show existence
of a sectorial region G of opening larger than π/k and a function
f ∈ H(G, C ) with f (z) ∼1/k f (z) in G.
96 5. Integral Operators

4. For
“(±1 + s1 u) · . . . · “(±ν + sν u)
m(u) = ,
“(β1 + σ1 u) · . . . · “(βµ + σµ u)
with positive parameters ±j , βj , sj , σj restricted by j=1 sj =
j=1 σj , determine the radius of convergence of the series

m(n) z n .
k(z) =

Moreover, show that k(z) admits holomorphic continuation along ev-
ery ray other than the positive real axis.
Summable Power Series

In this chapter we shall present Ramis™s concept of k-summability of for-
mal power series [225, 226]. We shall also study the classical de¬nition of
moment summability methods in a form suitable for application to formal
power series and relate these methods to k-summability. For some historical
remarks, see Chapter 14.
A general summability method may be viewed as a linear functional S
on some linear space X of sequences, or equivalently, series, with complex
entries. In many cases the functional has the following representation: Let

an in¬nite matrix A = [ajk ]∞
j,k=0 be given. Then we say that a series 0 xk

is A-summable if the series k=0 ajk xk converge for every j ≥ 0 and

xk = lim ajk xk

exists. In this case the number SA ( xk ) is called the A-sum of the series
xk . Such summability methods are called matrix methods, and we say
that the series is summable by the method A, or for short, is A-summable.
The space X consisting of all A-summable series is called the summability
domain of the method. Observe that this terminology gives good sense even
xk with xk in a Banach space E .
in the more general situation of series
One may even replace the entries ajk of the matrix A by (continuous) linear
operators on E , but we shall not consider this here.
For our purposes it is more natural to replace the index j by a continuous
parameter T . So instead of an in¬nite matrix A we have a sequence of
functions ak (T ), T ≥ 0, which we again denote by A. Then the functional
98 6. Summable Power Series

has the form SA ( xk ) = limT ’∞ k=0 ak (T ) xk . If the series we want

to sum is a formal power series, hence xk = fk z k , then k=0 ak (T ) fk z k ,
in case of convergence for, say, |z| < ρ, de¬nes a family of holomorphic
functions a(z; T ) on D = D(0, ρ). It then is natural to restrict ourselves to
cases where convergence for T ’ ∞ is locally uniform in z, at least for z in
some subregion G of D, so that we obtain a holomorphic function f (z) =
limT ’∞ a(z; T ) on G. In this case we say that the formal power series f (z) is
A-summable on the region G, and we shall refer to the function (SA f )(z) =
f (z) as the sum of f (z), in the sense of the summation method A. Aside
from the holomorphy of the sum, however, we need more properties of our
summability method in order to make it suitable for di¬erential equations:
Suppose we know that the formal series f (z) satis¬es some linear di¬erential
equation, and we have seen earlier that the radius of convergence of such a
formal solution can be zero. If f (z) is indeed summable to f (z) on G, we
should like to conclude from general properties of the summation method
that f (z) also solves the di¬erential equation. Moreover, the origin should
be a boundary point of G, and we should like to conclude that f (z) is
asymptotic to f (z), perhaps of some Gevrey order s > 0. Thus, in order
to make a summability method suitable for formal solutions of ODE, we

• that its summability domain X is a di¬erential algebra, containing
all convergent power series,

• that the functional S is a homomorphism, and so is not only linear,
but maps products to products and derivatives to derivatives, and

• that the map J, de¬ned on p. 67, inverts S.


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