Number Theory Problem No.2

Given the Diophantine equation :

a 2 + b 2 + c 2 = 2041 a^{2} + b^{2} + c^{2} = 2041

How many ordered integer ( a , b , c ) (a,b,c) solutions exist?

384 256 128 320 768 448 512 192

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2 solutions

Chew-Seong Cheong
Aug 20, 2020

Since brute force is to be used, we must well have computer doing the work. Using the Code environment of Brilliant.org:

Alright. But can you use mathematical approach to solve this problem?

Anh Khoa Nguyễn Ngọc - 9 months, 3 weeks ago

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Nope. I can assume a a is even and start with a = 0 , 2 , 4 , 22 a=0, 2, 4, \cdots 22 . It is going to be very long.

Chew-Seong Cheong - 9 months, 3 weeks ago

By using logic and brute force, we have 9 trios: ( 45 , 4 , 0 ) , ( 42 , 14 , 9 ) , ( 41 , 18 , 6 ) , ( 40 , 21 , 0 ) , ( 39 , 22 , 6 ) , ( 39 , 18 , 14 ) , ( 36 , 27 , 4 ) , ( 36 , 24 , 13 ) , ( 32 , 24 , 21 ) . (45,4,0), (42,14,9), (41,18,6), (40,21,0), (39,22,6), (39,18,14), (36,27,4), (36,24,13), (32,24,21).

Now it's time for combinatorics.

Since the problem asked for the integer solutions, we have to include both positive- and negative-containing solutions. Of the given result, we see that there are 7 all-nonzero trios, which can be shuffled (For example: ( 45 , 4 , 0 ) (45,4,0) and ( 4 , 45 , 0 ) (4,45,0) or ( 0 , 4 , 45 ) (0,4,45) are different) in 3 ! 3! ways and have 2 3 2^{3} combinations of positive or negative value for each trios.

For the remaining two, we also have 3 ! 3! ways to shuffle but only 2 2 2^{2} positive-or-negative-value combinations since 0 0 is a neutral number (it's neither positive nor negative).

The answer is 7 × 3 ! × 2 3 + 2 × 3 ! × 2 2 = 384 7 \times 3! \times 2^{3} + 2 \times 3! \times 2^{2} = \boxed{384} different solutions.

You should have put the option wisely . The answer will always divisible by 6 and 8Anyone can just check the divide the option by 6 and guess the answer

Himanshya Bhardwaj - 9 months, 3 weeks ago

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You cannot know if there are solutions with the same number occuring multiple times in the triplet without solving the equation in some form.

Jesse Nieminen - 9 months, 3 weeks ago

Well, I don't think so. Guessing these kinds of problem is difficult because they involve in combinatorics.

Also I gave 3 on 8 answer that are divisible by 24 so that the probability of guessing the correct one is 1 3 \frac{1}{3} , which make it more difficult (Unless you use online website like Wolfram|Alpha).

Anh Khoa Nguyễn Ngọc - 9 months, 3 weeks ago

Anh Khoa, I have changed it to the proper word "ordered" .

Chew-Seong Cheong - 9 months, 3 weeks ago

but how could we find the 9 sets? just by brute force? could we find out a more beautiful way

煜杰 王 - 8 months ago

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As all I know, there are no other method for these kinds than using logic and brute force.

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