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Turkish Journal of Computer and Mathematics Education Vol.12 No.11 (2021), 2240-2242

Research Article

2240

Study Of Hermite-Fejer Type Interpolation Polynomial

Mousa Makey Khrajan

1

1Department of Mathematics and Computer Applications College of Science, University of AL_Muthanna, Iraq 1mmkrady@gmail.com mmkrady@mu.edu.iq

Article History: Received: 10 January 2021; Revised: 12 February 2021; Accepted: 27 March 2021; Published

online: 10 May 2021

Abstract: Given 𝒇 ∈ 𝑪 [−𝟏, 𝟏] and n points (node) in [−𝟏, 𝟏], the Hermite-Fejer type (HFT) interpolation polynomial is the polynomial of degree at most (2n-1) that agree with 𝒇 and has zero derivative at each of the nodes. The aim of this paper is to investigate HFT interpolation polynomial of n such that n is an even number of Chebyshev of the first kind. Mathematics Subject classification: 2010 primary 41A05, Secondary 41A10

Keywords: Lagrange interpolation, polynomial interpolation, Berman’s phenomenon.

1. Introduction

Suppose that an function 𝒇(𝒙) are continuous in [−𝟏, 𝟏] denoted by C; 𝒇 ∈ 𝑪 [−𝟏, 𝟏], and let

𝑿 = {𝒌𝒌,𝒏}𝒌=𝟎𝒏=𝟏 , 𝒌 = 𝟎, 𝟏, 𝟐, … , 𝒏 = 𝟏, 𝟐, 𝟑, … …(1)

be an infinite triangular matrix of nodes such that, for all n

−𝟏 ≤ 𝒙𝒏−𝟏,𝒏< ⋯ < 𝒙𝟏,𝒏< 𝒙𝟎,𝒏 ≤ 𝟏 … … … . . (𝟐)

The well known Lagrange interpolation polynomial of 𝒇 is the polynomial 𝑳𝒏(𝑿, 𝒇)(𝒙) = 𝑳𝒏(𝑿, 𝒇, 𝒙) of

degree at most (n-1) which satisfies

𝑳𝒏(𝑿, 𝒇, 𝒙𝒌,𝒏) = 𝒇(𝒙𝒌,𝒏); 𝒌 = 𝟎, 𝟏, … , 𝒏 − 𝟏

we further denote by 𝑯𝒏(𝒇, 𝑿. 𝒙), the polynomial of degree 2n-1 that is uniquely determined by the following

conditions

𝑯𝒏(𝒇, 𝑿, 𝒙𝒌𝒏) = 𝒇(𝒙𝒌𝒏); 𝑯′𝒏(𝒇, 𝑿, 𝒙𝒌𝒏) = 𝟎 ,

𝒌 = 𝟎, 𝟏, 𝟐, … , (𝒏 − 𝟏) and 𝒙𝒌,𝒏≡ 𝒙𝒌

The process {𝑯𝒏(𝒇, 𝑿, 𝒙𝒌)}𝑛=0∞ is called a Hermite-Fejer Type interpolation polynomial (HFT).

Faber showed that [1] for any X there exists 𝒇 ∈ 𝑪 [−𝟏, 𝟏] so that 𝑳𝒏(𝑿, 𝒇, 𝒙) does not converge uniformly to

𝒇 on [−𝟏, 𝟏] as → ∞ .

Let the points {𝒙𝒌𝒏} are the roots of the n-th Chebyshev nodes of the first kind T = { 𝒙𝒌𝒏= 𝐜𝐨𝐬 (

𝟐𝒌+𝟏

𝟐𝒏 ) 𝝅 ; k=0,1, …,(n-1) ; n=1,2,3,…}………… (3)

Where Chebyshev polynomial defined as 𝑻𝒏(𝒙) = 𝐜𝐨𝐬(𝒏 𝒂𝒓𝒄 𝐜𝐨𝐬 𝒙), |𝒙| ≤ 𝟏

This result states that if the modulus of continuity 𝝎(𝜹, 𝒇) of f is defined by 𝝎(𝜹) = 𝝎(𝜹, 𝒇) = 𝑺𝒖𝒑|𝒙−𝒚 |≤𝜹 {|𝒇(𝒙) − 𝒇(𝒚)| } , this value 𝝎(𝜹) is said to be

Modulus of continuity of the function f(x) , then 𝑳𝒏(𝑻, 𝒇) converges uniformly to f with 𝝎 ( 𝟏

𝒏 , 𝒇) 𝐥𝐨𝐠 𝒏 →

𝟎 as 𝒏 → ∞.

Ageneralization of Lagrange interpolation is provided by Hermite –Fejer interpolation process. Given a non- negative integer m and nodes X defined by [1,2], the HFT interpolation polynomial 𝐇𝒎,𝒏(𝑿, 𝒇)(𝒙) =

𝐇𝒎,𝒏(𝑿, 𝒇, 𝒙) of f is the unique polynomial of degree at most (m+1)(n-1) which satisfies the (m+1)(n)

conditions: 𝐇𝐦,𝐧(𝑿, 𝒇, 𝒙𝒌𝒏) = f(𝒙𝒌𝒏) ; 0≤ 𝒌 ≤ 𝒏 − 𝟏

𝐇𝒎,𝒏(𝒓)(𝑿, 𝒇, 𝒙𝒌,𝒏) = 𝟎 ; 𝟏 ≤ 𝒓 ≤ 𝒎, 𝟎 ≤ 𝒌 ≤ 𝒏 − 𝟏

J. BYRNE and J.SMITH [8] focus on an aspect of HFT that has become known as Berman's phenomenon

occurs if the Chebyshev nodes are augmented by the end point of [-1,1], that is for the case of nodes

𝒙𝒌,𝒏+𝟐≡𝒙𝒌= 𝐜𝐨𝐬(

𝟐𝒌+𝟏

𝟐𝒏 )𝝅 ,𝐤=𝟏,𝟐,…,𝐧

𝒙𝟎,𝒏+𝟐≡𝒙𝟎=𝟏 ; 𝒙𝒏+𝟏,𝒏+𝟐≡𝒙𝒏+𝟏=−𝟏 } … … … . (𝟒)

Obtained by adding the nodes ∓𝟏 to the node (3) .D.L.Berman[1] it is show that process constructed for

f(x)=|𝒙| diverges at x=0 , while in [2] he showed that for f(x)=x2 , the process

𝐇𝟏,𝒏(𝑻∓𝟏, 𝐟, 𝟎)diverges every where in (−𝟏, 𝟏). An explanation for Berman's phenomenon was provided by

(2)

Turkish Journal of Computer and Mathematics Education Vol.12 No.11 (2021), 2240-2242

Research Article

2241

Theorem: (Bojanic) [5].If f ∈ 𝒄[−𝟏, 𝟏 ]has left and right derivatives 𝒇𝑳′(𝟏) and 𝒇𝑹′(−𝟏) at 1 and -1 ,

respectively, then 𝐇𝟏,𝒏(𝑻∓𝟏, 𝐟) converges uniformly to f on [-1,1] if and only if 𝒇𝑳′(𝟏)= 𝒇𝑹′(−𝟏)=0.Cook and

Mils[6] in 1975, who showed that if f(x)= (𝟏 − 𝒙𝟐)𝟑 then 𝐇

𝟑,𝒏(𝑻∓𝟏, 𝐟, 𝟎) diverges. The result in [6] later

extended by my paper [7] that showed 𝐇𝟑,𝒏(𝑻∓𝟏, 𝐟, 𝐱) diverges at each point in (-1,1). Byrne and Smith [8]

investigate Berman's phenomenon in the set of (0,1,2)

HFI, where the interpolation polynomial agree with f and vanishing first and second derivatives at each

node.G.Mastroianni and I.Notarangelo [9] study the uniform and LP convergence of Hermite and Hermite-Fejer interpolation.

It is obvious that when n is odd, the nodes 𝒙𝒌,𝒏≡ 𝒙𝒌𝒏= 𝐜𝐨𝐬 ( 𝟐𝒌+𝟏

𝟐𝒏 ) 𝝅 include the point x=0 . Therefore it

will be assume that n is an even number (say n=2m) and this the aim of paper. Consider the matrix of nodes

𝒙𝒌𝟐𝒎= 𝐜𝐨𝐬 ( 𝟐𝒌−𝟏

𝟒𝒎) 𝝅 ,k=1,2,…,2m ; x=0 , m=1,2,… …………..(5)

and study Hermite – Fejer Type(HFT) interpolation polynomial constructed at these nodes of degree 4m+1 is a uniquely determined by the following conditions:

𝑯𝟐𝒎(𝒇, 𝑿, 𝟎) = 𝒇(𝟎); 𝑯𝟐𝒎′ (𝒇, 𝑿, 𝟎) = 𝟎

𝑯𝟐𝒎(𝒇, , 𝟎) = 𝒇(𝟎); 𝑯𝟐𝒎′ (𝒇, , 𝟎) = 𝟎 , k=1,2,…2m

Therefore 𝑯𝟐𝒎(𝒇, 𝑿, 𝒙) can be written as:

𝑯𝟐𝒎(𝒇, 𝑿, 𝒙) = ∑ 𝒇(𝒙𝒌) 𝒙𝟐 𝒙𝒌𝟐 𝟐𝒎 𝒌=𝟏 𝑻𝟐𝒎𝟐 (𝒙)(𝟏−𝒙𝒌 𝟐) 𝟒𝒎𝟐 (𝒙−𝒙 𝒌)𝟐 [ 1- 𝟐−𝒙𝒌𝟐 𝒙𝒌 (𝟏−𝒙𝒌 𝟐 ) (𝒙 − 𝒙𝒌)] + 𝒇(𝟎) 𝑻𝟐𝒎 𝟐 (𝒙). --- (6)

Theorem :The HFT interpolation polynomial { 𝑯𝟐𝒎(𝒇, 𝑿, 𝒙)} constructed with the matrix (5) for :

(i) f (x)= x2 is convergent at all points of (-1,1).

(ii) f (x)= x is divergent for all points x≠ 𝟎 in (-1,1).

2. Technical Preliminaries

We shall quite frequently make use the following results before proof theorem[7] Lemma: (i) ∑ 𝟏 (𝟏−𝒙𝒌 𝟐 ) 𝒏 𝒌=𝟏 = 𝒏𝟐 (ii) ∑ 𝟏 (𝟏+𝒙𝒌)= ∑ 𝟏 (𝟏−𝒙𝒌)= 𝒏 𝟐 𝒏 𝒌=𝟏 𝒏 𝒌=𝟏 (iii) ∑ 𝟏 𝒙𝒌𝟐 𝒏 𝒌=𝟏 = 𝒏𝟐 (iv) ∑ 𝟏 (𝟏+𝒙𝒌)𝟐= ∑ 𝟏 (𝟏−𝒙𝒌)𝟐= 𝟐𝒏𝟒+𝒏𝟐 𝟑 𝒏 𝒌=𝟏 𝒏 𝒌=𝟏 (v) ∑ 𝟏 (𝟏−𝒙𝒌𝟐)𝟐 𝒏 𝒌=𝟏 = 𝒏𝟒+𝟐𝒏𝟐 𝟑 (vi) ∑ 𝒙𝒌𝟐 (𝟏−𝒙𝒌𝟐)𝟐= 𝒏𝟒−𝒏𝟐 𝟑 𝒏 𝒌=𝟏 (vii) ∑ 𝟏 𝒙𝒌𝟒 𝒏 𝒌=𝟏 = 𝒏𝟒+𝟐𝒏𝟐 𝟑 . 3. Proof of theorem

For f (x)= x2 , the formula (6)becomes

𝑯𝟐𝒎(𝒛𝟐, 𝑿, 𝒙) ≡ 𝑯𝟐𝒎(𝒛𝟐, 𝒙) = 𝒙𝟐 𝒍 𝒌 𝟐 𝟐𝒎 𝒌=𝟏 (𝒙) − 𝒙𝟐 ∑ (𝟐−𝒙𝒌 𝟐) 𝒙𝒌 (𝟏−𝒙𝒌𝟐) 𝟐𝒎 𝒌=𝟏 𝒍𝒌𝟐(𝒙)(𝒙 − 𝒙𝒌) --- (7) Where 𝒍𝒌(𝒙) = 𝐓𝒏(𝒙) 𝐓𝒏′(𝒙𝒌)(𝒙−𝒙𝒌) and 𝑻𝒏(𝒙) = 𝑻(𝒙) = ∏ (𝒙 − 𝒙𝒌) 𝒏

𝒌=𝟏 be Lagrange interpolation polynomial.

According to Fejer's result , when |𝒙| ≤ 𝟏

𝐦𝐤=𝟏𝐥𝐤𝟐(𝐱) → 𝟏 𝐚𝐬 𝐦 → ∞ --- (8)

From (7) & (8) it follows that the equation 𝒍𝒊𝒎

𝒎→∞𝑯𝟐𝒎( 𝒛

𝟐 , 𝒙) = 𝒙𝟐 is equivalent to the equation:

𝐥𝐢𝐦 𝐦→∞ (𝟐−𝐱𝐤 𝟐) 𝐱𝐤 (𝟏−𝐱𝐤 𝟐) (𝐱 − 𝐱𝐤) 𝐥𝐤 𝟐 𝐦 𝐤=𝟏 (𝐱) = 𝟎 , |𝐱| ≤ 𝟏 ---(9)

It can be proved that if 𝐱 = 𝐜𝐨𝐬 𝜽, then ∑ 𝐥𝐤𝟐 (𝐱) 𝐱𝐤 𝟐𝐦 𝐤=𝟏 = 𝟏 𝐱 [𝟏 − 𝐬𝐢𝐧 𝟒𝐦𝛉 𝐜𝐨𝐬 𝛉 𝟒𝐦 𝐬𝐢𝐧 𝛉 ] + 𝟏+𝐱𝟐 𝐱𝟐 𝐓𝟐𝐦(𝐱)𝐓𝟐𝐦′ (𝐱) 𝟒𝐦𝟐 ---(10) From (8)&(10) we can get (9).

To prove (ii), we indicate the proof according to (6) , for f (x)=x , we have 𝐇𝟐𝐦(𝐳, 𝒙)= 𝐱𝟐∑ 𝐥𝐤𝟐(𝐱) 𝐱𝐤 𝟐𝐦 𝐤=𝟏 − 𝐱𝟐 𝐓𝟐𝐦𝟐 (𝐱) 𝟐𝐦𝟐 ∑ 𝟏 𝐱𝐤 𝟐(𝐱−𝐱𝐤) 𝟐𝐦 𝐤=𝟏 + 𝐱𝟐 𝐓𝟐𝐦𝟐 (𝐱) 𝟒𝐦𝟐 𝟏 (𝐱−𝐱𝐤) 𝟐𝐦 𝐤=𝟏

(3)

Turkish Journal of Computer and Mathematics Education Vol.12 No.11 (2021), 2240-2242

Research Article

2242

Since ∑ 𝟏 𝒙𝒋𝟐 𝟐𝒎

𝒋=𝟏 = 𝟒𝒎𝟐 , we can deduce from this that

𝐇𝟐𝒎(𝒛, 𝒙) = 𝒙 [𝟏 − 𝐬𝐢𝐧 𝟒𝒎𝜽 𝐜𝐨𝐬 𝜽 𝟒𝒎 𝐬𝐢𝐧 𝜽 ] (𝟏 + 𝒙 𝟐)𝐜𝐨𝐬 𝟒𝒎𝜽 𝟒𝒎 𝐬𝐢𝐧 𝜽− 𝟐𝒙 𝐓𝟐𝒎 𝟐 (𝒙) −𝐓𝟐𝐦(𝐱)𝐓𝟐𝐦′ (𝐱) 𝟐𝒎𝟐 + 𝒙 𝟐( 𝐓𝟐𝐦(𝐱)𝐓𝟐𝐦′ (𝐱) 𝟒𝒎𝟐 ) ---(11)

By the lemma in [5]&[7] for any 𝒙 ∈ (−𝟏, 𝟏) there exists a sequence of {𝟐𝒎𝒌}𝒌=𝟏∞ such that

𝐥𝐢𝐦

𝒌→∞𝐓𝟐𝒎𝒌

𝟐 (𝒙) = 𝟏. Therefore it follows from (11), that

𝐥𝐢𝐦

𝒌→∞𝐇𝟐𝒎𝒌(𝒛, 𝒙) = −𝒙.

Therefore the sequence diverges at every points if 𝒙 ≠ 𝟎 𝑖𝑛 (−𝟏, 𝟏).

4. Conclusion

To Construct HFT interpolation polynomial which converges for 𝒇(𝒙) = 𝒙𝟐 in (-1,1) , while diverges for

𝒇(𝒙) = 𝒙 𝒇𝒐𝒓 𝒂𝒍𝒍 𝒙 ≠ 𝟎 𝒊𝒏 (−𝟏, 𝟏) 𝑤ℎ𝑒𝑟𝑒 𝒏 is an even integer number at the node of degree 4m+1. References

1. D.L.Berman, On the theory of interpolation, Dokl.Akad.Nauk SSSR,163(1965), PP.551-554 (in Russian) .

2. D.L.Berman, An investigation of Hermite-Fejer interpolation process , Dokl.Akad.Nauk SSSR, 187(1969),PP.241-244.

3. D.L.Berman, An Hermite-Fejer interpolation process which divergeseverywhere , Izv ,Vyssh,Mate. no.1, pp.3-8 ,1970.

4. D.L.Berman, Investigation of interpolation system of Chebyshev nodes , Dokl,AnSSSR,Vol. 176, no.2,pp . 239-242,1967.

5. R.Bojanic , Necessary and sufficient conditions for the convergence of the extended Hermite-Fejer interpolation process , Acta Math.Acad.sci.Hungar.,36 (1980),pp.271-279.

6. W.L.Cook and T.M.Mills, On Berman's phenomenon in interpolation theory ,Austral.Math.Soc.,12 (1975),pp.457-465.

7. M.Makey, On an extended Hermite-Fejer interpolation process ,J.of Al-qadisyah for pure science,13(2008)n0.2 ,pp.1-8 .

8. Graeme J. Byrne and Simon J. Smith,On Berman's phenomenon for (0,1,2)Hermite-Fejer interpolation , J.Numer.Anal.Approx.Theory, vol.48 (2019) no.1,pp.3-15.

9. G.Mastroianni And I.Notarangelo, Hermite and Hermite-Fejer interpolationat Pollaczek zeros. X Jean Conference on Approximation, ubeda, June 30 th – july 5 th,2019 .

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