Are you smarter than me? 41

Find the number of integer solution(s) to the equation x 2 + y 2 + z 2 = 2016 x^2+y^2+z^2=2016 .


The answer is 144.

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1 solution

Discussions for this problem are now closed

Aditya Raut
Dec 24, 2014

First of all, note that 2016 0 ( m o d 4 ) 2016 \equiv 0 \pmod{4} implies that none of x , y , z x,y,z is odd.

Because if any out of x , y , z x,y,z is odd, x 2 , y 2 , z 2 x^2,y^2,z^2 will correspondingly be congruent to 1 1 ( m o d 4 ) \pmod 4 , and no combination of 3 or less 1s will give a number divisible by 4.

Hence we conclude 2 x , 2 b , 2 c 2 \mid x , 2\mid b , 2\mid c


We can say a = x 2 , b = y 2 , c = z 2 a=\frac{x}{2} , b = \frac{y}{2} , c = \frac{z}{2} , and number of solutions of a 2 + b 2 + c 2 = 2016 4 a^2+b^2+c^2 = \frac{2016}{4} will be same as of original equation.

a 2 + b 2 + c 2 = 504 a^2+b^2+c^2 = 504 .

Now because 4 504 4 \mid 504 , we conclude that again all of a , b , c a,b,c are even.


So now problem reduces to integer solutions of j 2 + k 2 + l 2 = 126 j^2+k^2+l^2 = 126

As 4 126 4 \nmid 126 , we conclude that exactly \textbf{exactly} 2 out of j , k , l j,k,l are odd . And, the original numbers x , y , z x,y,z are basically 4 j , 4 k , 4 l 4j,4k,4l .


For simplicity, first let's find positive integer solutions such that a b c a\leq b\leq c .

Now by observation, we have only the following solutions

( 1 , 2 , 11 ) ( 1 , 5 , 10 ) ( 3 , 6 , 9 ) (1,2,11) \\ (1,5,10) \\ (3,6,9)

For ordered triples, there will be 3 × 3 ! 3\times 3! of these solutions.

Now as we want integer solutions, negatives would work too! Each of the 3 can be either +ve or -ve, giving 2 × 2 × 2 = 8 2\times 2\times 2 = 8 combinations of +ve and -ve.

Hence answer is 3 × 3 ! × 8 = 3 × 6 × 8 = 144 3\times 3! \times 8 = 3\times 6\times 8 = \boxed{144} .

A programming approach, which gives the 18 18 positive triples, and then we can think of the -ve and multiply by 8.

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Aditya Raut - 6 years, 5 months ago

in which class you r ? @Aditya Raut

Karan Shekhawat - 6 years, 5 months ago

Used the same method.You Will get exact triplets if you use intial value of i,j,k<=-45.

Prakash Chandra Rai - 6 years, 5 months ago

Exactly the same!

Ryan Tamburrino - 6 years, 5 months ago

Exactly the same but I was wondering if there is a generating function solution too.

Kartik Sharma - 6 years, 5 months ago

The generating function solution would be to try and find the coefficient of X 2016 X^{2016} in ( X 0 + 2 X 1 + 2 X 4 + 2 X 9 + ) 3 ( X^ 0 + 2 X^1 + 2X^4 + 2X^9 + \ldots ) ^3 . Unfortunately, there is no simply way to simplify the expression (that I know of), so we're back to square 1.

Calvin Lin Staff - 6 years, 5 months ago

This sum is expressible in terms of the Jacobi theta function , but I'm not sure that helps at all.

Michael Lee - 6 years, 5 months ago

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