QUASI-CONVEX UNIVALENT FUNCTIONS

I nt. J. Math. Math.

Si.

Vol.

No.

2

(1980) 255-266

²⁵⁵

QUASI-CONVEX UNIVALENT FUNCTIONS

K.

INAYAT NOOR

and

D.K. THOMAS

Kerman University P. O. Box 182

Kerman, Iran

University College of Swansea SWANSEA SA2 8PP, Wales

(Received May 9, 1979 and

n

Revised form in June 25, 1979)

ABSTRACT. In this paper, a new class of normalized univalent functions is intro- duced. The properties of this class and its relationship with some other sub- classes of univalent functions are studied. The functions in this class are close- to-convex.

1980

MATHEMATICS SUBJECT CLASSIFICATION CODES:

Primary OA2;

Secondary

0A34

KEFWORDS AND PHRASES.

Univalent

functions,

Quasi-nvex, Close-to-nvex.

1 INTRODUCTION

Denote by S the class of functions f which are regular and univalent in the

,

unit disc E and satisfy f(0) ⁰ and f’ (0) i. The subclasses S and C of star- like and convex functions respectively are well known and have been extensively

,

studied. S and C are connected by the basic property

,

f C if and only if zf’ e S (i.i)

The subclass K of S consisting of close-to-convex function is also well

,

known and many of the properties of S can be extended to the wider class K.

The purpose of this paper ^is to introduce a natural analogue of the class C

in terms of the property defined in (i.i).

2. MAIN RESULTS.

Def. Let f be regular ⁱⁿ

E

with f(O) 0 and f’(0) I. Then f is said to be quasi-convex in

E

if there exists a convex function g with g(0) 0, g’(0) 1 such that for z e

E,

Re

.(zf’

(z))’

g’ (z)"

^{> 0. (2.2)}

Denote the class of quasi-convex functions by Q.

It is clear that when f(z) g(z), C Q so that C c Q. We show first that Q ^c K, so that every quasi-convex functions is univalent.

THEOREM I. Let f e Q. Then, for z e

E,

Re

__zf..’.(z).

_{> 0}

g(z)

and so Q ^c K ^c S thus, every quasi-convex function is close-to-convex and hence univalent in

E.

PROOF: A result of Libera [4] shows that, if s and t are functions re-

,

gular in E with s(0) t(0) 0 and t e S then for z E,

Re

__s.’_(z)

_{> 0 + Re} s(z)

> O.

t’(z) t(z)

An immediate application of this with s(z) zf’ (z) and t(z) g(z) proves

QUASI-CONVEX UNIVALENT FUNCTIONS 257

Theorem i.

It follows at once from the definition that

f Q if and only if zf’ e K. (2.2)

We can thus write

C

>

^S

Q

>

^K

where the direction of the arrow indicates set inclusion.

Theorem i shows that the image domain for all f e Q is close-to-convex.

However a specific characterisation of the image domain for f e Q remains an open question.

we

state now some basic properties of quasi-convex functions which can easily be extended from the class of convex functions. We omit the proofs as they are simple extensions from the convex case.

THEOREM 2. Let f e Q with f(z) z

+

a_nzn Then for

zl

^{r < i,}

n=2

(i)

lanl

^{< i, n 2,3,...,}

(+/-i) _2-^<

If’(z)

^< ₂

(l+r) (l-r)

(iii) ^{r <_}

If ^(z)

^-< _l-rr l+r

(iv) w > 1/2 where

f(z)+

^{w in E}

All inequalities are sharp, equality being attained for

f0(z)

l-zZ

We now give an example of a function in Q which is not convex.

Example 1 3 1 Let

fl

^{be the} Koebe function; i e

fl(z)

_(-z)^z 2 Then

fl

maps E i-i conformally onto the w-plane cut from- to along the

x+

z

negative real axis. Let

f2:f2

^"(z) _l+xz ^{x E. Then}

f2

^{maps E onto it-}

self and takes the origin onto the point x. Define

f3

^by

f3(z) fl[f2(z)] fl(x) f3(0)

^{0 and}

f3"

^{(0) 0 (since}

f3

^is univalent), and let

f4

f3

^(z)

f4

^(z)

f’0---

^{z E.}

Combining all these transformations, we can write

fl ^{+_z )_ fl(x}

F(z)

+

xz

f

^(x)

^(-Ix i)

z (i- z)²

The function F is close-to-convex. In fact,

fl/X+

^z

F’(z)

1 ^{+ .z/}

fl(x)

⁽ⁱ

⁺ x--z)2

F"(z)

F’

(z)

(i

+

xz)

and

OUASI-CONVEX

^{UNIVALENT FUNCTIONS}

zF"(z)

F’

(z)

,,!x ⁺

^z

1

x+z

i

+z

^f

^x+z I

^{(x+z) (i +xz)}

2xz

z(, .- ^1,* lz).

(x+ z) (i +xz) +i

,,/x+z

fl )("

^z(-

xl +

,/" ^+_

^z

)l (-,-z)" (,,,z)

fl [l+xz

^/

(x+z) (t+xz)

i+

)z

fl

^(x+z)

^(l+x--z)

_(x+z)

_(.iz)

Le

ie i8 ²

ie x

+

re

i

^{re (i-}

z re r

I ^{e d8} dq).

is

)

1

+

x re (x

+

re (i

+

xre

For Now,

and

2

^with

⁽ⁱ

^<

2

^{), we} have correspndlng

81

^{and 8}2 with

(I

^ie

F"(rei)

} ( f’()} ^rlei

Re

+

re d8 Re i+ d

;

F’

re

18) fl

⁽⁾

’i+ rlei ^f-[ (rlei@)}

Re

----’) d

fl

Hence, for 0 ₁ ^and

81

^{< 82,}

i8

F_’:(

^re

iS)I

^d8

F’

(re

i8)

02

_i

_fM(rl ei

Re 1+

rle

I

fl (rlei )i

^{de >}

-

which shows that f e K.

Now,

+ x+

l+z z)

F’

(z)

l+xz 1

+

^l+x

l+xZ!

i- l-x

(i -x)3 i (i +x)

(l+xz)²

(1

+

(i z)³

l+x

,

i- x

=z

B=I+x ^1-’x

Intergrating, we have for z e E,

’F(z)

z(l

+

--

^z)

(i z)

We notice that F maps E onto the w-plane cut along a half-line

".

nce

the choice of the point x in E is arbitrary, we can select x in such a way that the half-line does not pass through the origin in F(E), ^which

means F is not, ⁱⁿ general, starlike. Because of relationships (3) and (1) between the classes Q and K and S we conclude that, in general,

QUASI-CONVEX UNIVALENT FUNCTIONS 261 2

the function

f,

^{defined in E by}

f,(z)

^{F ()} d ^{belongs to Q but not to} 0

C.

3. SOME GROWTH PROBLEMS

Clunle and Keogh [i] showed that, if f C with f(z) z

+

n=2

.

^{a z}n ^and fCE) has finite area. Then n a o(I) as n ^/ and the exponent Is best pos-

n

slble. We extend this result to quasl-convex functions.

THEOREM 3. Let f Q with f(z) z

+

^a zn If f(E) has finite area, then n=2 ⁿ

n a o(1) as n +

,,

^the index of n being 5est possible.

n

PROOF: We use a modified version of themethod of Clunie and Pommerenke [2].

By (2), we can write

(zf’(z))’ g’(z)h(z),

where Re h(z) ^{)0 for z} E and h(0) i. Thus,

z(zf’(z))’ 2zg’(z) Reh(z)

zg’

(z) h(z), and so with z rei8 0 ^< r < i, Cauchy’s formula gives for n > I

2 1

n a z(zf’

(z))’e-nSd8

n 21[rn 0

21[ _in0

d

1 zg’(z)Re h (z)e O-

1[rn ₀ Since Re h(z) ^{> 0 for a} E,

21[

n21a

_n ^{-< i}_n

J ^[zg’(z) IRe

^{h(z) dO}

⁺

ⁱ

1[r 0 21[rn

21[

I _zg

2rⁿ 0

’(z) h(z)e-in0dO.

21[

’(z) h(z)e^In0 dO (3.2)

Re[z(zf,(z)),e-larg zg’(z)]

from (3.1), and so inte- grating the first of the above two integrals by parts we have

1

[zg’(z) IRe

^{hCz)dO Re 1 z(zf’Cz))}

^’e-larg

^z

^{g’ (z)de}

--

2/[ ^{-i arg z}

^{g’ (Z)de}

^e

Re

----I

_{zf’(z)e z g}

wrn 0

(z))

Also, zg’(z) h(z)

z(zf,(z)),e-21

^{arg z g’(z)} and so (3.2) and (3.3) give 2

n

[a n]

^-<

--n

_r ^{Re f’(z)e -i arg g}

’(z) ^do(arg

^{Z g’(z))}

i 2r2n

2/[

-2i arg z

g’(Z)do

zn+l

(’f’

(z))’e 0

1 1

II

+ -12

^say.

n 2n

r

(3.4)

To estimate

II,

^{we note that, since f is regular in E and the area of}

f(E) ^is finite, M(r,f’)

0.(i)

_{as r- i}

1 -r

2

where M(r,f)

xl(reiS.) ^I-

^{Since d}0 (arg z g’(z)) 2, we have 0

o(I) as r + i.

I1

^1-r

OUASI-CONVEX

UNIVALENT FUNCTIONS 263

Integrating

12

^{by parts gives}

12

^{2 Fn(z)e-2i arg z}

^{g’ (Z)Re(zg’ (z))’}

^d0

0 g’(z)

where

z

Fn(Z)

^tn (tf’(t))’dt

zn+if’(z) -nfn(Z),

0

and

z

f (z)

tnf

’(t)dt.

0

Now

r

IF

n (z) ^<

rn+iM(

tn

r,f’)

+

nM(r,f’) dt 0

<

2rn+iM(r,

f’).

2w

dO 2, we have

(z’

(z))_’

Since Re

(z) g 0

12

^<

8rn+iM(r,f,)_. o(I)i

-r as r + i as before.

Finally, choosing r 1- ⁱ in (3.4) the estimates for I

I ^and

12

^give

n

na o(i) as n ^/ and Theorem 3 ^is proved.

n

An examination of the proof of Theorem 3 gives

COROLLARY: Let f e Q, Er {z:

Izl

^{r < i} and A(r)} be the area of

f(Er).

Then, for n ^> 2,

n[anl

^{0(i) A(I}

^1_)%

n (3.5) We remark that (3.5) holds for the class S

,

but appears still to be an open

problem for the class K.

Denote by C(r) the closed curve which is the image of f(E

r)

^{and by L (r) the}

length of C(r). We prove

THEOREM 4. Let f e Q. Then, for 0 ^< r ,e i,

2/(A(r))

^< L(r) ^< 2/

(A(/r))

(io

l/_r

Further, if A(r) < for 0 < r < i, then

L(r) o(i)

(lo_ir)1/2

^{as r +. I.}

PROOF: The left hand inequality follows atonce. from the isoperlmetrlc in- equality. Since f e Q, F(z) zf’ (z) is close-to-convex. Thus from [3,p.45]

2 2

LCr)

[zf

^(z)

ld0 ^[(z);d0

0 0

r

< 2 M(;),zf’)

d;)

0

n=l

n=l

ⁿ

^/

i-1/2

2/(A(/r)

io

ig_r

QUASI-CONVEX UNIVALENT FUNCTIONS 265

If a(r) < for 0 ^< r < I, then from (3.7)

L(r)--" 2

[ la

_n

Ir

ⁿ

n=l N

< 2 _n=l

[ lanlr

ⁿ

⁺

_n=N

^[ nlanl

2n 1/2

21/2 r

n

n=l N

2 _n=l

[ lanlrn ⁺

^N

^logl#r2

1 1/2

where

N

^{/ 0 as N / Thus L(r) o(i)}

Io

^{"as r /i.}

1 1 1/2

The convex function

fl

^(z)

^log_z

^{shows that the factor}

lOl-r

ⁱⁿ

(3.6) ^{is best} possible.

For f e C it is well know that L(r) ^< 2M(r). It follows from (3.6) that for f e Q, L(r) 0(i) M(r)

log_r

^{as r/l. The question} of whether the factor

io

^{can be} removed remains open.

In conclusion, we remark that other results for the class C can be extended to quasi-convex functions, often with only minor alterations in the proof. The objective of this paper has been to introduce the class Q, exhibit its basic properties and give some results whose proofs are not trivial extensions from the class C.

REFERENCES

i. CLUNIE,J.G. and KEOGH, F.R., "On starlike and convex schlicht functions", J. London Math.Soc., 35 (1960), 229-233.

2. CLUNIE, J.G. and POMMERENKE, Ch., "On the coefficients of close-to-convex univalent functions", J. London Math.Soc., 41 (1966),161-165.

3. HAYMAN, W.K., ’Multivalent functions’ Cambridge, 1967

4 LIBERA, R.J "Some classes of regular univalent functions" Proc.Amer Math Soc., 16 (1965), 755-758.

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