Lower bound in the Bernstein inequality for the first derivative of algebraic polynomials

We prove that $\max |p′(x)|$, where $p$ runs over the set of all algebraic polynomials of degree not higher than $n ≥ 3$ bounded in modulus by 1 on [−1, 1], is not lower than \( {{\left( {n - 1} \right)} \mathord{\left/{\vphantom {{\left( {n - 1} \right)} {\sqrt {1 - {x^2}} }}} \right.} {\sqrt {1 -...

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Дата:2009
Автори: Podvysotskaya, A. I., Подвысоцкая, А. И.
Формат: Стаття
Мова:Російська
Англійська
Опубліковано: Institute of Mathematics, NAS of Ukraine 2009
Онлайн доступ:https://umj.imath.kiev.ua/index.php/umj/article/view/3053
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Ukrains’kyi Matematychnyi Zhurnal
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author Podvysotskaya, A. I.
Подвысоцкая, А. И.
Подвысоцкая, А. И.
author_facet Podvysotskaya, A. I.
Подвысоцкая, А. И.
Подвысоцкая, А. И.
author_sort Podvysotskaya, A. I.
baseUrl_str https://umj.imath.kiev.ua/index.php/umj/oai
collection OJS
datestamp_date 2020-03-18T19:44:07Z
description We prove that $\max |p′(x)|$, where $p$ runs over the set of all algebraic polynomials of degree not higher than $n ≥ 3$ bounded in modulus by 1 on [−1, 1], is not lower than \( {{\left( {n - 1} \right)} \mathord{\left/{\vphantom {{\left( {n - 1} \right)} {\sqrt {1 - {x^2}} }}} \right.} {\sqrt {1 - {x^2}} }} \) for all $x ∈ (−1, 1)$ such that \( \left| x \right| \in \bigcup\nolimits_{k = 0}^{\left[ {{n \mathord{\left/{\vphantom {n 2}} \right.} 2}} \right]} {\left[ {\cos \frac{{2k + 1}}{{2\left( {n - 1} \right)}}\pi, \cos \frac{{2k + 1}}{{2n}}\pi } \right]} \).
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fulltext K O R O T K I P O V I D O M L E N N Q UDK 517. 5 A. Y. Podv¥sockaq (Kyev nac. un-t ym. T. Íevçenko) OCENKA SNYZU V NERAVENSTVE S. N. BERNÍTEJNA DLQ PERVOJ PROYZVODNOJ ALHEBRAYÇESKYX MNOHOÇLENOV We prove that max ( )′p x , where p runs over all algebraic polynomials of degree not greater than n ≥ 3 with modules bounded by 1 on −[ ]1 1, , is not less than (n – 1) / 1 2− x for all x ∈ (–1, 1) such that x ∈ cos , cos ( ) / 2 1 2 1 2 1 20 2 1 k n k nk n + − +   = −[ ] π π∪ . Dovedeno, wo max ( )′p x , de p probiha[ mnoΩynu vsix obmeΩenyx za modulem odynyceg na −[ ]1 1, alhebra]çnyx polinomiv stepenq ne vywe n ≥ 3, ne menße (n – 1) / 1 2− x dlq vsix tyx x ∈ (–1, 1), dlq qkyx x ∈ cos , cos ( ) / 2 1 2 1 2 1 20 2 1 k n k nk n + − +   = −[ ] π π∪ . 1. Vvedenye y osnovnoj rezul\tat. Pust\ Pn oboznaçaet mnoΩestvo vsex alhebrayçeskyx mnohoçlenov s dejstvytel\n¥my koπffycyentamy stepeny ne v¥ße n ∈N : = {1, 2, … }, C I( ) — klass vsex dejstvytel\noznaçn¥x neprer¥v- n¥x funkcyj f na I : = −[ ]1 1, , f C I( ) : = max ( )x I f x∈ y πn : = p pn C I∈ ≤{ }P ( ) 1 , K p( ) : = x I p x∈ ={ }( ) 1 , p n∈π . Esly p n∈π y card K(p) = n + 1, to suwestvugt takye znaky σ1, σ2 ∈ {+ 1, – 1}, çto p x( ) = σ σ1 2T xn ( ) , hde card A oboznaçaet çyslo razlyçn¥x πlementov nekotoroho koneçnoho mnoΩestva A ⊆ R1 , a T xn( ) : = cos n arccos x — mnoho- çlen Çeb¥ßeva pervoho roda n-j stepeny. Yzvestno [1, s. 99], çto vo mnohyx πkstremal\n¥x zadaçax na mnoΩestve πn πkstremum dostyhaetsq na odnom yz mnohoçlenov p n∈π , dlq kotoroho n ≤ ≤ card K(p) ≤ n + 1. Odnoj yz takyx zadaç qvlqetsq tesno svqzannaq s yzvestn¥m neravenstvom Bernßtejna ′p x( ) ≤ n x p C I 1 2− ( ), x ∈ (– 1, 1), p n∈P , (1) πkstremal\naq zadaça naxoΩdenyq M xn( ) : = max ( ) p n p x ∈ ′ π (2) dlq proyzvol\noj fyksyrovannoj toçky x I∈ (sm. [2, s. 32], zadaça 1.1). ∏ta zadaça b¥la reßena v 1892 h. V. A. Markov¥m [3] (sm. takΩe [2, s. 38]), kotor¥j dokazal, çto M xn( ) = ′T xn( ) , esly x prynadleΩyt specyal\n¥m pod¥nterva- lam α βj j,[ ]{ } yntervala I. Ym b¥lo dokazano takΩe, çto na ostavßyxsq pod¥ntervalax I znaçenye M xn( ) dostyhaetsq na tak naz¥vaem¥x polynomax Zolotareva, t.@e. na tex mnohoçlenax p n∈π \ πn−1, dlq kotor¥x card K(p) = n. © A. Y. PODVÁSOCKAQ, 2009 ISSN 1027-3190. Ukr. mat. Ωurn., 2009, t. 61, # 5 711 712 A. Y. PODVÁSOCKAQ Neposredstvenno yz opredelenyq mnoΩestva πn y neravenstva Bernßtejna (1) sledugt takye svojstva funkcyy M xn( ): M xn( ) = M xn( )− , x I∈ ; 1 ≤ M xn( ) ≤ n x1 2− , x ∈ (– 1, 1). (3) Yspol\zuq nov¥e svojstva mnohoçlenov Zolotareva, ustanovlenn¥e v [4], doka- Ωem sledugwug ocenku snyzu funkcyy M xn( ) . Teorema. Dlq proyzvol\noho n ≥ 3 ymeet mesto neravenstvo M xn( ) ≥ n x − − 1 1 2 , x ∈   k n k n k n= [ ]− + − +   0 2 1 2 1 2 1 2 1 2 / cos ( ) , cos∪ π π . (4) 2. Predvarytel\n¥e rezul\tat¥. Budem yspol\zovat\ oboznaçenyq dlq mnohoçlenov Zolotareva, vvedenn¥e v [4, s. 154]. Dlq kaΩdoho natural\noho n ≥ 2 y dejstvytel\noho çysla 1 ≤ b < ∞ suwestvuet edynstvenn¥j mnohoçlen Zolotareva Z xn b, ( ) stepeny n so svojstvamy ′ −Z bn( ) = 0, Z bn( )− 1 = Z bn( )− 2 = 1, (5) b2 > b1 ≥ b ≥ 1, b1 = b1(b), b2 = b2(b), dopuskagwyj analytyçeskoe predstavlenye (sm. [4], formula (11)) Z xn b, ( ) = cos ( )n xω( ) , x I∈ , (6) ω( )x : = ω( , )x b : = x t b t dt 1 21∫ − ρ( , ) , ρ( , )x b : = ( ) ( ) ( ) b x b b x b b x + +( ) +( ) 2 1 2 . Pry πtom Z xn, ( )1 = T xn   cos2 2 π n     + sin2 2 π n       y v dopolnenye k formulam (6) budem yspol\zovat\ oboznaçenye (sm. [4], formula (5)) Z xn b, ( ) : = T x b n b n n +     +               tg tg 2 2 2 1 2 π π , b ∈[ ]0 1, . (7) V pryvedennom nyΩe dokazatel\stve teorem¥ budet yspol\zovano sledugwee svojstvo funkcyy ρ( , )x b . Lemma. Pust\ 1 < b < ∞ y funkcyq ρ( , )x b opredelena formuloj (6). Tohda ρ( , )x b ≥ 1 1 2 −   n , x ∈[ ]0 1, . (8) Dokazatel\stvo. MnoΩestvo πn qvlqetsq v¥pukl¥m y Zn b n, ∈π pry lgbom b ≥ 0. Poπtomu dlq proyzvol\n¥x θ ∈[ ]0 1, , b ≥ 1 y ε < b ( ) ,1 − θ Zn b + θ εZn b, + = Zn b, + θ ε( ), ,Z Zn b n b+ − ∈ πn . (9) PredpoloΩym, çto y I∈ y Z yn b, ( ) = σ ∈ {1, – 1}. Tohda yz (9) sleduet σ εZ y Z yn b n b, ,( ) ( )+ −( ) ≤ 0, ε < b, b ≥ 1. (10) V [5, s. 12 – 14] poluçeno v¥raΩenye mnohoçlenov Zn b, , b ≥ 1, çerez πllypty- çeskye funkcyy, yz kotoroho sleduet, çto vse koπffycyent¥ mnohoçlena Zn b, , ISSN 1027-3190. Ukr. mat. Ωurn., 2009, t. 61, # 5 OCENKA SNYZU V NERAVENSTVE S. N. BERNÍTEJNA DLQ PERVOJ … 713 kak y funkcyy b1(b) y b 2 (b), qvlqgtsq neprer¥vno dyfferencyruem¥my funkcyqmy parametra b. Poπtomu razdelyv neravenstvo (10) na ε y perexodq k predelu pry ε → 0, poluçym σ ∂ ∂b Z yn b, ( ) ≤ 0, esly ε > 0, y σ ∂ ∂b Z yn b, ( ) ≥ 0, esly ε < 0, otkuda sleduet y I∈ , Z yn b, ( ) = 1 ⇒ ∂ ∂b Z yn b, ( ) = 0. (11) Sohlasno yzvestn¥m svojstvam mnohoçlena Zn b, pry b > 1 (sm. [4, s. 153]) Zn b, ( )±1 = 1 y Z yn b, ( ) = 1 dlq vsex (n – 2)-x kornej uravnenyq ′Z yn b, ( ) = 0, kotor¥e raspoloΩen¥ v (– 1, 1), (n – 1)-j koren\ πtoho uravnenyq raven – b. Poπtomu yz (11) sleduet, çto dlq kaΩdoho b > 1 suwestvuet takaq nenulevaq dejstvytel\naq postoqnnaq λ( )b , çto ∂ ∂b Z xn b, ( ) = λ( ) ( ),b x x b Z xn b 1 2− + ′ , b > 1. (12) Dlq naxoΩdenyq λ( )b podstavym predstavlenye (6) v (12): − −∫∂ ∂ ρ b t b t dt x 1 21 ( , ) = λ ρ ( ) ( , ) b x x b x b x 1 1 2 2 − + − = λ( ) ( ) ( ) b x x b x b 1 2 1 2 − + + , y posle dyfferencyrovanyq po x budem ymet\ 1 1 2− x b x b∂ ∂ ρ( , ) = λ ∂ ∂ ( ) ( ) ( ) b x x x b x b 1 2 1 2 − + + . (13) Poskol\ku ρ( , )x b = (x + b) / ( ) ( )x b x b+ +1 2 , oboznaçyv ′bk : = ′b bk ( ) , k = = 1 2, , poluçym 1 1 2− x b x b∂ ∂ ρ( , ) = x b x x b x b x b b x b b x b + − + + + − ′ + − ′ +    2 1 2 2 1 2 1 1 2 2( ) ( ) ( ) , ∂ ∂x x x b x b 1 2 1 2 − + +( ) ( ) = 1 2 1 1 1 1 1 1 12 1 2 1 2 − + + + − − − + − +     x x b x b x x x b x b( ) ( ) y yz (13) najdem ( )x b x b b x b b x b + + − ′ + − ′ +     2 1 1 2 2 = = λ( ) ( )b x x x x b x b 1 1 1 1 1 1 12 1 2 − + + − − + − +     . Otsgda s uçetom ravenstv ( )x x x x b x b 2 1 2 1 1 1 1 1 1 1− + + − − + − +     = ISSN 1027-3190. Ukr. mat. Ωurn., 2009, t. 61, # 5 714 A. Y. PODVÁSOCKAQ = ( ) ( ) ( ) ( ) ( ) x b x x b b x x b 2 1 1 2 2 1 1 1 1 1 − − + + + + − +     = ( )( )b x x b 1 1 1 1− − + + + ( )( )b x x b 2 2 1 1+ + + = b1 + b2 – ( )( )1 11 1 1 + − + b b x b + ( ) ( )b b x b 2 2 2 1 1+ − + = = b1 + b2 – b x b 1 2 1 1− + – b x b 2 2 2 1− + poluçym 2 – ′b1 – ′b2 + b b x b b1 1 1 − + ′ + b b x b b2 2 2 − + ′ = λ( )b b x b b x b b b1 2 1 2 2 2 1 2 1 1− + + − + − −      . Takym obrazom, dolΩn¥ ymet\ mesto ravenstva ′b1 + ′b2 – 2 = λ( ) ( )b b b1 2+ , ′ −b b b1 1( ) = λ( )b b1 2 1−( ), ′ −b b b2 2( ) = λ( )b b2 2 1−( ). Podstavlqq v pervoe ravenstvo znaçenyq proyzvodn¥x yz vtoroho y tret\eho, poluçaem λ( ) ( )b b b1 2+ = λ( )b b b b 1 2 1 1− − + λ( )b b b b 2 2 2 1− − – 2, otkuda s uçetom neravenstv b2 > b1 > b > 1 (sm. (5)) naxodym 2 λ( )b = b b b 1 2 1 1− − – b1 + b b b 2 2 2 1− − – b2 = bb b b 1 1 1− − + bb b b 2 2 1− − = = ( )( ) ( )( ) ( )( ) bb b b bb b b b b b b 1 2 2 1 1 2 1 1− − + − − − − > 0, t.@e. λ( )b = 2 1 1 1 2 1 2 2 1 ( )( ) ( )( ) ( )( ) b b b b bb b b bb b b − − − − + − − > 0 ∀ >b 1. (14) Teper\ zametym, çto znak pravoj çasty (13) pry x ∈[ ]0 1, sovpadaet so znakom v¥raΩenyq 1 1x + – 1 1 − x – 1 1x b+ – 1 2x b+ = − − 2 1 2 x x – 1 1x b+ – 1 2x b+ < 0, poπtomu s uçetom (13) y (14) ymeem ∂ ∂ ρ b x b( , ) < 0 ∀ >b 1, x ∈[ ]0 1, . (15) No v [3, s. 65] dokazano, çto lim ,b n bZ→+∞ = Tn−1, a πto v sylu formul (6) ozna- çaet, çto lim ( , )b x b→+∞ ρ = ( )n − 1 2 / n2 pry kaΩdom x ∈ −[ ]1 1, . Poπtomu na os- novanyy (15) poluçaem ρ( , )x b ≥ ( )n − 1 2 / n2 dlq proyzvol\n¥x b > 1 y x ∈ [0, 1], çto y trebovalos\ dokazat\. 3. Dokazatel\stvo teorem¥. Pust\ n ≥ 2, 0 ≤ k ≤ n/2[ ] – 1, αk n( ) : = : = cos 2 1 2 k n + y βn : = tg2 2 π n     . Poskol\ku T nn kα ( )( ) = 0, po opredelenyg (7) ISSN 1027-3190. Ukr. mat. Ωurn., 2009, t. 61, # 5 OCENKA SNYZU V NERAVENSTVE S. N. BERNÍTEJNA DLQ PERVOJ … 715 mnohoçlen Zn b, na promeΩutke cos ( ) 2 1 2 1 k n + −   π, cos 2 1 2 k n +   π ymeet rovno odyn nul\ αk n( ) @( )1 + b nβ – b nβ , kotor¥j ub¥vaet ot αk n( ) do αk n( ) @( )1 + βn – βn , kohda b vozrastaet ot 0 do 1. Poπtomu dlq kaΩdoho x ∈ αk n( )[ ( )1 + βn – βn , αk n( )] suwestvuet takoe b ∈[ ]0 1, , çto koren\ αk n( ) @( )1 + b nβ – b nβ mnoho- çlena Zn b, sovpadaet s x. No yz (7) sleduet, çto ′Z xn b, ( ) = n x b n1 1 12− + β , otkuda ymeem 1 2− ′x n Z xn b, ( ) ≥ 1 1 + βn = cos2 2 π n ≥ 1 – π2 24n ≥ 1 – 1 n , çto dokaz¥vaet neravenstvo (4) dlq vsex x ∈ αk n( )[ ( )1 + βn – βn , αk n( )]. Rassmotrym sluçaj, kohda v (4) x ∈ αk n( )−[ 1 , αk n( ) ( )1 + βn – βn) . Dlq proyzvol\noho b ≥ 1 oboznaçym nuly polynoma Zn b, çerez αr n n( ), 0 ≤ ≤ r ≤ n – 1, pryçem (sm. [4, s. 155]) αn b n−1( ) < αn b n−2( ) < …. < α1 b n( ) < α0 b n( ) < 1. V [3, s. 65] dokazano, çto kohda b vozrastaet ot 1 do + ∞, αk b n( ) ub¥vaet ot αk n( ) (1 + βn) – βn do αk n( )− 1 , pryçem lim ,b n bZ→+∞ = Tn−1. Poπtomu dlq proyzvol\noho x ∈ αk n( )−( 1 , αk n( ) ( )1 + βn – βn) suwestvuet takoe b > 1, çto αk b n( ) = x, a v πtoj toçke yz formul¥ (6) sleduet ravenstvo ′Z xn b, ( ) = n x x 1 2− ρ( ) , yz kotoroho vsledstvye (8) v¥tekaet neravenstvo (4). Esly Ωe x = αk n( )− 1 , to neravenstvo (4) neposredstvenno sleduet yz ravenstva ′−Tn 1 αk n(( – 1)) = (n – – 1) / 1 1 2− −αk n( ) . Çetnost\ funkcyy M xn( ) (sm. (3)) zaverßaet dokaza- tel\stvo teorem¥. 1. Tyxomyrov V. M. Nekotor¥e vopros¥ teoryy approksymacyy. – M.: Yzd-vo Mosk. un-ta, 1976. – 303 s. 2. Bojanov B. D. Markov-type inequalities for polynomials and splines // Approxim. Theory X: Abstract and Classical Anaysis. – Vanderbilt Univ. Press, 2002. – P. 31 – 90. 3. Markov V. A. O funkcyqxæ, naymenæ uklonqgwyxsq otæ nulq væ dannom promeΩutkæ. – Sankt Peterburhæ, 1892. 4. Avvakumova L. S. Comparison of integral functionals depending on the second derivative of Che- byshev and Zolotarev polynomials // East J. Approxim. – 1999. – 5, # 2. – P. 151 – 182. 5. Zolotarev E. Y. Polnoe sobranye soçynenyj. T. 2. – L.: Yzd-vo AN SSSR, 1932. – T. 2. – 364 s. Poluçeno 29.09.08 ISSN 1027-3190. Ukr. mat. Ωurn., 2009, t. 61, # 5
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spelling umjimathkievua-article-30532020-03-18T19:44:07Z Lower bound in the Bernstein inequality for the first derivative of algebraic polynomials Оценка снизу в неравенстве С. Н. Вернштейна для первой производной алгебраических многочленов Podvysotskaya, A. I. Подвысоцкая, А. И. Подвысоцкая, А. И. We prove that $\max |p′(x)|$, where $p$ runs over the set of all algebraic polynomials of degree not higher than $n ≥ 3$ bounded in modulus by 1 on [−1, 1], is not lower than \( {{\left( {n - 1} \right)} \mathord{\left/{\vphantom {{\left( {n - 1} \right)} {\sqrt {1 - {x^2}} }}} \right.} {\sqrt {1 - {x^2}} }} \) for all $x ∈ (−1, 1)$ such that \( \left| x \right| \in \bigcup\nolimits_{k = 0}^{\left[ {{n \mathord{\left/{\vphantom {n 2}} \right.} 2}} \right]} {\left[ {\cos \frac{{2k + 1}}{{2\left( {n - 1} \right)}}\pi, \cos \frac{{2k + 1}}{{2n}}\pi } \right]} \). Доведено, що max $\max |p′(x)|$, де $p$ пробігає множину всіх обмежених за модулем одиницею на [-1,1] алгебраїчних поліномів степеня не вище $n ≥ 3$, не менше \( {{\left( {n - 1} \right)} \mathord{\left/{\vphantom {{\left( {n - 1} \right)} {\sqrt {1 - {x^2}} }}} \right.} {\sqrt {1 - {x^2}} }} \) для всіх тих $x ∈ (−1, 1)$ для яких \( \left| x \right| \in \bigcup\nolimits_{k = 0}^{\left[ {{n \mathord{\left/{\vphantom {n 2}} \right.} 2}} \right]} {\left[ {\cos \frac{{2k + 1}}{{2\left( {n - 1} \right)}}\pi, \cos \frac{{2k + 1}}{{2n}}\pi } \right]} \). Institute of Mathematics, NAS of Ukraine 2009-05-25 Article Article application/pdf https://umj.imath.kiev.ua/index.php/umj/article/view/3053 Ukrains’kyi Matematychnyi Zhurnal; Vol. 61 No. 5 (2009); 711-715 Український математичний журнал; Том 61 № 5 (2009); 711-715 1027-3190 rus en https://umj.imath.kiev.ua/index.php/umj/article/view/3053/2855 https://umj.imath.kiev.ua/index.php/umj/article/view/3053/2856 Copyright (c) 2009 Podvysotskaya A. I.
spellingShingle Podvysotskaya, A. I.
Подвысоцкая, А. И.
Подвысоцкая, А. И.
Lower bound in the Bernstein inequality for the first derivative of algebraic polynomials
title Lower bound in the Bernstein inequality for the first derivative of algebraic polynomials
title_alt Оценка снизу в неравенстве С. Н. Вернштейна для первой производной алгебраических многочленов
title_full Lower bound in the Bernstein inequality for the first derivative of algebraic polynomials
title_fullStr Lower bound in the Bernstein inequality for the first derivative of algebraic polynomials
title_full_unstemmed Lower bound in the Bernstein inequality for the first derivative of algebraic polynomials
title_short Lower bound in the Bernstein inequality for the first derivative of algebraic polynomials
title_sort lower bound in the bernstein inequality for the first derivative of algebraic polynomials
url https://umj.imath.kiev.ua/index.php/umj/article/view/3053
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