Gelatinous Polynomials

Algebra Level 5

A polynomial p ( x ) p(x) is called Gelatinous if its coefficients are integers and p ( 100 ) = 100 p(100)=100 . If p ( x ) p(x) is a Gelatinous polynomial, what's the maximum number of integer solutions k k for p ( k ) = k 3 p(k)=k^3 ?


The answer is 10.

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1 solution

Abhishek Sinha
May 10, 2016

Consider the polynomial g ( x ) = p ( x ) x 3 g(x)=p(x)-x^3 . We want to find maximum number of integral roots of g ( x ) g(x) subject to the given constraints on the polynomial p ( x ) p(x) . By the fundamental theorem, the polynomial g ( x ) g(x) can be factorized into its roots (assumed to be there are m m of them) as follows g ( x ) = p ( x ) x 3 = A i = 1 m ( x α i ) g(x)=p(x)-x^3= A \prod_{i=1}^{m} (x-\alpha_i) For some real number A A and possibly complex numbers α i , i = 1 , 2 , , m \alpha_i, i=1,2, \ldots, m . Since p ( 100 ) = 100 p(100)=100 , we require 100 10 0 3 = A i = 1 m ( 100 α i ) 100-100^3= A\prod_{i=1}^{m}(100-\alpha_i) i.e., A i = 1 m ( 100 α i ) = 1 × 2 2 × 3 2 × 5 2 × 11 × 101 A\prod_{i=1}^{m}(100-\alpha_i)=-1\times 2^2\times 3^2 \times 5^2 \times 11 \times 101 Clearly, there can be at most m m integral roots of g ( x ) g(x) which is achieved when A A and all of α i \alpha_i 's are integers. In that case the polynomial p ( x ) p(x) will have all integral coefficients. From the factorization above, we can choose at most 2 + 2 + 2 + 1 + 1 + 2 = 10 2+2+2+1+1+2=10 (the last two is for the factor ± 1 \pm 1 ) distinct integral values of the roots α i \alpha_i , which gives the maximum number of integral solutions for p ( k ) = k 3 p(k)=k^3 .

Great great! Stunning approach.

Mateo Matijasevick - 5 years, 1 month ago

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