Let f=a_0x^n+a_1x^(n-1)+...+a_(n-1)x+a_n in RR [X] a polynomial which has the roots contained in (−1,1). Prove that: |(a_1+a_3+a_5+…)/(a_0+a_2+a_4+…)|<1

zakownikbj

zakownikbj

Answered question

2022-09-25

Let f = a 0 x n + a 1 x n 1 + + a n 1 x + a n R [ X ] a polynomial which has the roots contained in (-1,1). Prove that:
| a 1 + a 3 + a 5 + a 0 + a 2 + a 4 + | < 1

Answer & Explanation

crearti2d4

crearti2d4

Beginner2022-09-26Added 9 answers

Without loss of generality I will assume a 0 > 0. Let p = a 1 + a 3 + and q = a 0 + a 2 + . If n is even then
0 < f ( 1 ) = q p
0 < f ( 1 ) = q + p
and combining these equations we get q > 0 and | p q | < 1. If n is odd then
0 < f ( 1 ) = q p
0 < f ( 1 ) = q + p
and again q > 0 and | p q | < 1

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