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3. Compare Exercise 3 on p. 72.
Л†
5. Apply Theorem 35, with F = C , and T = Tn z n with Tn = 0 for
n в‰Ґ 1, and T0 f = f (П‰) for every f в€€ E .
256 Solutions

Section 6.4:
1. The series in Nos. 1, 2, and 5 are k-summable. The one in No. 3
converges for k в‰Ґ 1, and hence is k-summable in this case, while for
k < 1, there are inп¬Ѓnitely many singular directions. The one in No. 6
is 1-summable.
2. Proceed as in the proof of Lemma 10 part (b).
3. Verify directly that f (z; О») в€ј1 f (z; О») in |d в€’ arg z| < ПЂ; to do so,
=Л†
proceed as in the proof of Theorem 22 (p. 79).
в€ћ
mв€’1
Л† О“(О» + n) z n + z m 0 О“(О» + m + n) z n , for
4. Observe f (z; О») = 0
suп¬ѓciently large integer m, and use the previous exercise.
Л†Л† Л†Л†
5. Let g = S (B1 f ), g(В·; О±, ОІ) = S (B1 f (В·; О±, ОІ)). For Re ОІ > Re О± > 0,
use the Beta Integral (p. 229) to show
1
О“(ОІ)
(1 в€’ x)ОІв€’О±в€’1 xО±в€’1 g(xu) dx.
g(u; О±, ОІ) =
О“(О±) О“(ОІ в€’ О±) 0

Л†
Use this to conclude f (z; О±, ОІ) в€€ E {z}1 . To remove the restrictions on
Л†
О± and ОІ, use termwise integration resp. diп¬Ђerentiation of f (z; О±, ОІ).

Section 6.5:
1. Observe that the geometric series is contained in C {z}, for every
k > 0.
2. Assume f (z) в€ј1/k f (z) in a sectorial region of opening larger than
Л†
=
ПЂ/k, and let g = Lk (T в€’ f ), g = Lk (T в€’ f ). Conclude that g(z) в€ј1/k
Л†Л†Л†
Л† =
g (z) in another sectorial region of opening larger than ПЂ/k. This shows
Л†
that (О“(1+n/k)/m(n)) is a summability factor; the other case follows
similarly.
3. Follows from the previous exercise.
4. Use the previous exercise and Theorem 31 (p. 94).
5. Verify that f (z) в€ј1/k f (z) in G implies f (О»z) в€ј1/k f (О»z) in a corre-
Л† Л†
= =
Лњ
sponding sectorial region G of the same opening.
pв€’1
Л† Л†
fn z n , observe f (z Оµj ) =
6. For Оµ = exp[2ПЂi/p] and f (z) = j=0
p fpn z pn , and use the previous exercise.
7. Use termwise integration resp. diп¬Ђerentiation!
8. Use Exercise 1, and compare Exercise 1 on p. 99.
9. Use Theorem 37 (p. 106).
Solutions 257

Section 6.6:

4. Letting rn (z) denote the said Laplace transform, show п¬Ѓrst
П‰
rn+1 (z) = rn (z) в€’ n в‰Ґ 1.
rn (zП‰/[z + П‰]),
z+П‰

6. Show п¬Ѓrst that a power series in powers of z p belongs to E {z}k,d if
and only if it is in E {z}k,dj , for dj = d + 2jПЂ/p.

Section 7.1:

2. Show that it suп¬ѓces to restrict to |a| < Оµ.

Section 7.3:

1. Use Theorem 34 (p. 103), recalling that the notion of singular rays
implies that d0 + 2kПЂ are also singular, for every k в€€ Z.
Л† О“(1 + n/(2k)) z n + О“(1 + n/(2k)) (в€’1)n z n .
2. 2 f (z) =

Section 7.4:

2. For large z and suitable path of integration, write the integral in the
form
z
eв€’p(u)в€’О» log u (p (u) + О»/u) g (1/u) du,
О» p(z)
fj (1/z) = z e Лњ
в€ћ(2jПЂ/r)

with a function g analytic near the origin. Then, verify that one
Лњ
can always choose a path of integration along which the equation
t = p(u) + О» log u has a unique solution u = П€(t). Substitute the
integral accordingly and estimate.

4. Use Proposition 18.

Section 8.1:
(m) (m)
в‰¤ K n for every n, m в‰Ґ 1 (with suitably large
1. Recall that tn , bn
K > 0).
Лњ Лњ
2. (1 в€’ bx) dm+1 (x) = bx (dm (x) + dm (x)), (1 в€’ ax) dm+1 (x) = ax dm (x),
for every m в‰Ґ 0.
258 Solutions

Section 8.2:
1. Use Proposition 26 (p. 217).
2. Use the previous exercise.
3. Compare Proposition 13 (p. 105).

Section 8.3:
1. Set n = n0 and use Lemma 24 (p. 212) to conclude Tn0 = 0; then
increase n0 . For r = 0, A0 +n I and B0 have to have disjoint spectrum,
for every n в‰Ґ n0 .
Л†
2. Block rows, resp. columns, of T (z) according to the block structure
Л† Л†
of A(z), resp. B(z). Then, use the previous exercise and invertibility
Л†
of T (z) to show that to every j there corresponds a unique k so that
Лњ Л†
О»j = О»k . Permuting columns of T (z), conclude that one may assume
Л†
this to happen for k = j, so T (z) diagonally blocked. Use invertibility
once more to conclude that then sk = sk . For the convergence proof,
Лњ
use Exercise 3 on p. 128.
Л† Л†
3. Permute rows and columns of B(z), or of A(z), so that the leading
terms agree, which can be done according to the previous exercise.
Then in particular both matrices have the same block structure.

Section 8.4:
1. Observe that, owing to commutativity, (3.5) coincides with the re-
cursion formula for Tn .
2. Show that it suп¬ѓces to consider О› = diag [О»1 Is1 , . . . , О»Вµ IsВµ ] with
distinct О»m , and show that then N , hence: A, is diagonally blocked
in the block structure of О›. Next, use Lemma 24 (p. 212) to show that
B commutes with A if and only if it is likewise diagonally blocked,
so B certainly commutes with О›.
3. Use the series representation. In particular, conclude that T (z) com-
Л†
mutes with A(z).
4. Use the п¬Ѓrst exercise to reduce the problem to one where An = 0 for
n в‰Ґ r + 1. Then, use a constant transformation to put A0 into Jordan
canonical form. Next, use T (z) as in the previous exercise to remove
its nilpotent part, and conclude from above that then all other coef-
п¬Ѓcients are diagonally blocked in the block structure of A0 . Thus, it
suп¬ѓces to continue with one such block. Repeating these arguments,
conclude that every elementary system (3.3) can be transformed into
Solutions 259

one where the coeп¬ѓcient matrix has the desired form, except for the
conditions upon Re bm (0). This can then be arranged using shearing
transformations.

Л†
6. First, show that for a scalar formal meromorphic series t(z) the
Л† Л†
equation z t (z) = constant implies t(z) = constant. Then compute
Л† Л†
N1 T (z) в€’ T (z) N2 .

Section 8.5:

1. Build a vector x by rearranging the elements of X.

2. Holomorphicity follows from the results in Chapter 1. For the de-
terminant, show that for an arbitrary fundamental solution X2 (z) of
z x = A2 (z) x, the matrix X(z) X2 (z) is a solution of z x = A1 (z) x,
and use Proposition 1 (p. 6).

3. Use the previous exercise.

Section 9.1:

2. For a matrix C = Ejm with a 1 in position (j, m) and 0вЂ™s elsewhere,
в€’1
use the previous exercise to see that X1 (z) Ejm X2 (z) is a solution.
Check that all these solutions are linearly independent, and use The-
orem 2 (p. 6).

4. For (d), show (n, m) в€€ Suppj,n1 if and only if (9.2) holds in Sj =
Sj в€© . . . в€© Sj+n1 . Use this to conclude that C has the required form if
and only if Y (z) (I + C) Y в€’1 (z) в€ј1/k I in Sj . Then, use results from
=
Section 4.5. For (g), enumerate the data pairs in such a way that (9.2)
holds for djв€’1 + ПЂ/(2k) < arg z < dj + ПЂ/(2k) if and only if n < m,
so that Gj,n1 consists of upper triangularly blocked matrices. Then,
consider blocks directly above the diagonal п¬Ѓrst, next, treat those in
the next superdiagonal, etc.

Section 9.2:

1. Consider the identities obtained for the blocks of D, C+ , and Cв€’ in
a suitable order.

2. For the product on the right, п¬Ѓrst consider the blocks directly below
the diagonal, then in the next subdiagonal, and so forth. Also compare
this to Exercise 4 on p. 142.
260 Solutions

3. Observe that for integer values of k the number j0 always is even.
Then show that, after a suitable renumeration of the data pairs, i.e.,
a permutation of the columns of the HLFFS, we may assume that
(9.2) holds in the sector П„j < arg z < П„j+1 , if and only if n < m.
Use this to conclude than then the Vj+1 , . . . , Vj+j0 /2 are lower, the
Vj+j0 /2+1 , . . . , Vj+j0 upper triangular, and use the previous exercises.
Finally, use (9.4) to compute the remaining matrices.

Section 9.3:
1. Observe z Y (z) = B(z) Y (z), and use partial integration of (9.6).
2. Estimate analogously to the proof of Theorem 23 (p. 80).

Section 9.4:
2. For z0 в€€ Sjв€—(k) , observe the previous exercise to invert the integral
representation of the auxiliary functions. Then, deform the path of
integration in BorelвЂ™s transform. Finally, check that the integral does
not depend upon the choice of z0 .

Section 9.5:
2. Using Exercise 2 on p. 58, one can compute О¦1 (u; s; k), which is a
2-vector, with second component c (О»1 в€’ О»2 )в€’1 (u + О»2 )1в€’aв€’s F (1 +
О±, 1 + ОІ; 2 в€’ a в€’ s; z)/О“(2 в€’ a в€’ s), with z = (u + О»1 )/(О»1 в€’ О»2 ). Using
(k)
Exercise 6 on p. 27, one can then compute C21 . Assuming 2 в‰є 1, and
choosing arg(О»1 в€’ О»2 ) in dependence of k accordingly, one п¬Ѓnds
2ПЂi (О»1 в€’ О»2 )dв€’a
(k)
C21 = c.
О“(1 + О±) О“(1 + ОІ)

Section 9.6:
2. Verify that the spectrum of A0 is directly related to the data pairs of
HLFFS.

Section 9.7:
n+p n+p
2. Show 0 в‰¤ + xm ) в€’ 1 в‰¤ exp[ xm ] в€’ 1, for n, p в€€ N.
m=n (1 m=n

3. Proceed by induction with respect to m.
4. For n, p в€€ N, observe
Tn+p (z) в€’ Tn (z) = [I + Tn (z)] {[I + Un+p (z) В· . . . В· [I + Un+1 (z)] в€’ I},
and then estimate as in the previous exercises.
Solutions 261

Section 10.1:
1. Use the deп¬Ѓnition and justify interchanging the order of integration.
2. Let the jth integral be performed along arg z = dj ; then we may
choose djв€’1 so that 2|djв€’1 в€’ dj | в‰¤ Оєв€’1 .
j
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