A geometric property of the roots of Chebyshev polynomials
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(2) 2 kind cos = , sin 1 cos + 2 , 3 kind cos = cos 2 1 sin + 2 4ℎ kind (cos ) = , sin 2 1 kind. ∈ ℝ,. ∈ ℝ ∖ ℤ, ∈ ℝ ∖ 2ℤ + 1 ,. ∈ ℝ ∖ 2 ℤ,. for every ∈ ℕ ∪ {0}. For the reasons of cosmetic nature it is better to consider the re-scaling Chebyshev polynomials.
(3) 144. R. Wituła, E. Hetmaniok, D. Słota. ∗ (): = 2 , ∗ (): = , ∗ (): = , 2 2 2 and ∗ (): = , ∈ ℕ. 2. We note that polynomials ∗ () are called either Vieta-Lucas polynomials [5, 6] or Dickson’s polynomials [7]. Whereas ∗ () are called Vieta-Fibonacci polynomials [6]. Properties of algebraic and combinatoric nature of these polynomials are discussed for example in papers [5-11]. One of the spectacular properties of ∗ () are the following decompositions proved in [6]: ∗ () − ∗ ( + ) = ∗ () − − = = ( − − ), . (1). . where = exp ;. (−1) ∗ ( ) + ∗ ( + ) = (−1) ∗ ( ) + + = = ( − + ), . (2). . where = exp , for every ∈ ℕ, ∈ ℂ and ≠ 0. For example, we obtain the following special ones. ∗ (). − 2 cos 2 − 1
(4) = − 2 cos + . 2 , 2 − 1. ∗ ∗ () − (2 csc ) = ∗ () − tan − cot = 2 2 2 2 + 2 cot sin , = − 2 csc cos 2 − 1 2 − 1 . . for ≠ , ∈ ℤ,. (−1) ∗ ( ) + ∗ (2 sec !) = ! ! = (−1) ∗ ( ) + tan − + tan + = 4 2 4 2 (2 + 1) (2 + 1) = + 2 tan ! cos − 2 sec ! sin , 2 2 .
(5) A geometric property of the roots of Chebyshev polynomials. for ! ≠ + , ∈ ℤ,. 145. . (−1) ∗ ( ) + ∗ (coth !) = ! ! = (−1) ∗ ( ) + coth + tanh = 2 2 (2 + 1) (2 + 1) = + 2 csch ! cos − 2 coth ! sin , 2 2 . for ! ≠ 0.. Other fundamental properties, especially of an analytic nature, of polynomials ∗ , ∗ , ∗ and ∗ are presented in monographs [2, 4], see also interesting new results on a moment problem [12].. 1. Main result. Positive roots of polynomials ∗ , ∗ , ∗ and ∗ , respectively, are listed below [2]: , = 2 cos . (2 − 1) , 2 $, = 2 cos , +1 (2 − 1) , %, = 2 cos 2 + 1 2 (, = 2 cos , 2 + 1. = 1,2, … , " # , 2 = 1,2, … , " # , 2 +1 = 1,2, … , & ', 2 = 1,2, … , " # , 2. (we note that zeros , , = 1,2, … , " #, could also be deduced from (1) for . = exp and from (2) for = exp ).. We will present now our main result. Theorem. Let ∈ ℕ, ≥ 4, and ) , ) , … , ) denote the vertices of a regular. -gon * with the side of length . If = and () , )
(6) ) denotes the distance between ) and )
(7) , then we have (i). for 1 ≤ + ≤ " #, . ) , ) =. sin +. = cos . sin .
(8) 146. R. Wituła, E. Hetmaniok, D. Słota. (ii). 1 cos + + 2 ) , ) − ) , ) = = cos , cos 2. for 1 ≤ + + 1 ≤ " #, . (iii). () , ) ) − () , ) ) = 2 cos + = 2 (cos ),. for 2 ≤ + + 1 ≤ " #, . (iv). ) , ) = 2 , cos 2 − 1
(9) ,. for 2 ≤ 2+ ≤. . " #,. (v). ) , ) = 1 + 2 , cos 2 ,. for 1 ≤ 2+ + 1 ≤. " #.. . Corollary. By comparing the terms of sums from identities (%) and (%) with the roots of all four types of rescaling Chebyshev polynomials ∗ , ∗ , ∗ and ∗ , we obtain 0 , if = 2 , , .. () , ) ) = for 2 ≤ 2+ ≤ " # and . () , ) ) = for 1 ≤ 2+ + 1 ≤ " #. . 1. / . , %, if = 2 + 1, -. 01 + , ( if = 2 + 1, , .. 1. / .1 + , $, if = 2( + 1), .
(10) 147. A geometric property of the roots of Chebyshev polynomials. Proof of Theorem. If denotes the radius of the circle circumscribed on *, then sin ( ) and () , ) ) = , + ≤ " #. Hence by the law of sines we get = sin sin we obtain () , ) ) − () , ) ) =. sin + + 1
(11) − sin +. = sin 1 1 2 sin 2 cos + + 2 cos + + 2 = = = (cos ), sin cos 2. for 1 ≤ + + 1 ≤ " #, and next we have . () , ) ) − () , ) ) = = () , ) ) − () , ) ) + (() , ) ) − () , ) )) = 1 1 cos + + 2 + cos + − 2 = = cos 2 2 cos + cos 2 = 2 cos + = 2 (cos ), = cos 2. for 2 ≤ + + 1 ≤ " #. The last identity implies . () , ) ) = ,( () , ) ) − () , )() )) = 2 , cos 2 − 1
(12) ,. . . for 2 ≤ 2+ ≤ " #, and . () , ) ) = 1 + ,( () , ) ) − () , ) )) = 1 + 2 , cos 2 ,. . . for 2 ≤ 2+ + 1 ≤ " #, which ends the proof. . Conclusions In the paper we have described the new property of zeros of renormalized Chebyshev polynomials. It has been proven that the lengths of diagonals of a regular n-gon with the side of length equal to one are the sums of positive roots of the.
(13) 148. R. Wituła, E. Hetmaniok, D. Słota. respective renormalized Chebyshev polynomials of one from among four types. Formulae for decompositions of differences of values of the Chebyshev polynomials have been presented as well.. References [1]. Bialostocki A., Bialostocki D., Fluster M., Holman W., McAleer M., A geometric property of renormalized Chebyshev polynomials, College Math. J. (in review). [2] Mason J.C., Handscomb D.C., Chebyshev Polynomials, Chapman & Hall/CRC, New York 2003. [3] Paszkowski S., Numerical Applications of Chebyshev Polynomials and Series, PWN, Warsaw 1975 (in Polish). [4] Rivlin T., Chebyshev Polynomials from Approximation Theory to Algebra and Number Theory, 2nd ed., Wiley, New York 1990. [5] Robbins N., Vieta’s triangular array and a related family of polynomials, Internat. J. Math. Math. Sci. 1991, 14, 239-244. [6] Wituła R., Słota D., On modified Chebyshev polynomials, J. Math. Anal. Appl. 2006, 324, 321-343. [7] Bayad A., Cangul I.N., The minimal polynomial of 2 cos ( /) and Dickson polynomials, Appl. Math. Comput. 2012, 218, 7014-7022. [8] Knopfmacher A., Mansour T., Munagi A., Prodinger H., Staircase words and Chebyshev polynomials, Appl. Anal. Discrete Math. 2010, 4, 81-95. [9] Wituła R., Hetmaniok E., Słota D., Prodinger’s algebraic identities and their applications, Int. J. Pure Appl. Math. 2010, 64, 225-237. [10] Wituła R., Słota D., Decompositions of certain symmetric functions of powers of cosine and sine functions, Int. J. Pure Appl. Math. 2009, 50, 1-12. [11] Zhang Z., Wang J., On some identities involving the Chebyshev polynomials, Fibonacci Quart. 2004, 42, 245-249. [12] Castillo K., Lamblem R.L., Sri Ranga A., On a moment problem associated with Chebyshev polynomials, Appl. Math. Comput. 2012, 218, 9571-9574..
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