Are You a Good Critic?

Algebra Level 3

x 9 1024 x 8 + 512 x 7 256 x 6 + 128 x 5 64 x 4 + 32 x 3 16 x 2 + 8 x 4 = 0 x^9-1024x^8+512x^7-256x^6+128x^5-64x^4+32x^3-16x^2+8x-4 = 0

How many integer roots does the polynomial above have?

5 4 0 1 3 2 6 7

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2 solutions

Ralph Macarasig
May 30, 2016

When looking for integer roots, we must first determine if the polynomial is reducible over the integers. One of the well-known irreducibility criterion is the P e r r o n s Perron's I r r e d u c i b i l i t y Irreducibility C r i t e r i o n Criterion . The criterion states that for a polynomial P ( x ) P(x) ,

P ( x ) = x n + a n 1 x n 1 + a n 2 x n 2 + P(x) = x^n+ a_{n-1}x^{n-1} + a_{n-2}x^{n-2}+ . . . ... + a 1 x + a 0 + a_1x+a_0 with a 0 0 a_0 \neq 0

If a n 1 > 1 + a n 2 + a n 3 + |a_{n-1}| > 1+|a_{n-2}|+|a_{n-3}|+ . . . ... + a 1 + a 0 +|a_1|+|a_0| , then P ( x ) P(x) is irreducible over the integers.

Since in the given polynomial,

1024 > 1 + 512 + 256 + |-1024| > 1 + |512| + |-256| + . . . ... + 8 + 4 = 1021 + |8| + |-4| = 1021

1024 > 1021 1024 > 1021

Then, the polynomial is irreducible over the integers, hence, it has 0 \boxed 0 integer roots.

Useful link: Perron's Irreducibility Criterion starts on page 2 of this (It also contains the proof of this and as well as other criterion, I believe.)

Wow! From where did you learn Perron's irreducibility criterion? XD

Manuel Kahayon - 5 years ago

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From someone named Osbert! Do you know him? :))

Ralph Macarasig - 5 years ago
Alex G
May 30, 2016

Using rational root theorem, rational roots must be one of the following: 4 , 2 , 1 , 1 , 2 , 4 -4, -2, -1, 1, 2, 4 . The negative roots can easily be discarded, as all the terms will be negative if a negative number is plugged in. 1 is also easy to discard, as the sum of the coefficients are not 0. Plugging in 2 and 4 and evaluating results in a non zero number, meaning that there are no integer roots.

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