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A Sufficient Condition for Zeros (of a Polynomial) to be in the Interior of Unit Circle

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ANNALES UNIVERSITATIS MARIAE CU RIE-SKLODOWSK A LUBLIN-POLONIA

VOL XL1V, 4__________________________SECT1O A___________________________________1990

Department of Mathematics IndianInstitute of Technology, Kharagpur

V. K. JAIN

A Sufficient Condition for Zeros (of a Polynomial) to be in the Interiorof Unit Circle

Warunek dostateczny aby zerawielomianów leżały w kolejednostkowym

Abstract. The main result of thepaperis the followingtheorem: ifp(z) isa polynomial of degree n, with realcoefficients,havingall zeros withnon-positive real part and

p(r)<pW(îTïï)n (ïï)

for somer, R, 0 < r < R<1, then p(z) has at least(k + 1) zeros in |z| < 1.

Let p(z) = $2,"_0 aiz1 be a polynomial of degree n and let A/(p,r) = maxp|=r |p(z)|.

The following resultsconcerning thesize of M(p,r) arewellknown.

Theorem A [2]. If p(z)= aiz1 is a polynomial of degree n, then

(1-1) M(p,r) > M(p,R)

rn ~ R" o < r < R ,

with equality only for p(z) = Az".

Theorem B [1]. Ifp(z) = «i«' »•’ a polynomial of degree n, having no zeros in |z| < 1, then for 0< r <R < 1,

fi 91 M(p,r) M(p,R)

( (1 + r)" - (1+ R)" '

The result is best possible andequality holds forthe polynomial P(z') =

In this note weconsider certain restrictions on the estimate M(p,r) and obtain the information about the zerosofthe polynomialp(z)- More precisely, weprove

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20 V. K.Jain

Theorem. Let p(z) be apolynomial of degree n, with realcoefficients, having all zeros with non-positive real part. If, for some r, R (0 <r< R < 1),

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k, a non-negative integer, then p(z) has at least (k +1) zeros in |z| < 1. The result is best possible and the extremalpolynomialis

p(z)=(2+l)n-*-1Zk+1 .

Proof of the Theorem. Suppose p[z) has m zeros in |z| < 1 and m < k. Let p(z) =(z-z1)...(z-zm)(z-zm+i)...(z-zn) and assume |z>| < 1 (j = l,2,...,m).

Put

ÿ(z) = (2 - 2,)...(r-2m) ,

h(z) = (2 - 2m+l)...(2 - 2„) .

The polynomials p(z), g(z) and h(z) have positive coefficients. Hence, for all r, R (0 < r < H < 1),

(2.1) 9(r)>g(R)(^

by Theorem A,and

(2-2) h(r)>h(R)(^)"

by Theorem B.

On combining (2.1) and(2.2), we get

1>M =9(r)h(r) > 9(«)ft(fl)(—i)”"” (£) 1+ fl

)'

Mr)*

acontradiction, establishing the Theorem.

+r

For k = n — 1 and R = 1, we get

Corollary 1. Ifp(z) w a polynomial of degree n, with real coefficients, having allzeros with non-positive real part andif for some r, 0 < r < 1,

p(r)<p(l)(4Z)rn_1

then p(z) has all its zeros in |2| < 1.

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ASufficient Condition for Zeros (of a Polynomial)... 21 Wemay apply corollary I to thepolynomial znp(l/z)to get thefollowing Corollary 2. If p(z) is a polynomial withreal coefficients having all zeros with non-positive real part andiffor some R> 1

p(R) <p(l)—-

thenp(z) has no zeros in |z| < 1.

REFERENCES

[1] Govil , N. K., On the maximum modulus of polynomials, J.Math. Anal Appl. 112(1985), 253 258.

[2] Polya,G. , Szego , Problems and Theorems inAnalysis,Vol. 1, p.158, Problem III 269, Berlin 1972.

STRESZCZENIE

Głównym wynikiem tej pracy jest następujące twierdzenie: jeśli p(z) jest wielomianem o współczynnikachrzeczywistych,którego wszystkie zera lezą w domknięciulewej pólplaszczyzny oraz

P(r)<p(fl)(^)n (£)

dla pewnych r,R, 0 < r < R < 1, top(z) ma co najmniej (k+1) zer w kole|z|< 1

(receivedDecember20, 1990)

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