Is the following True or False?

Algebra Level 2

There exists four integers a , b , c , d 0 a, b, c, d \ge 0 such that: a 3 + b 3 + c 3 = d 2 a^3+b^3+c^3=d^2 Is the above statement true or false?

False True

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3 solutions

Chris Lewis
Jul 15, 2019

The simplest (and most boring) solution is just ( 0 , 0 , 0 , 0 ) (0,0,0,0) . A simple infinite family of solutions is ( 3 x 2 , 3 x 2 , 3 x 2 , 9 x 3 ) (3x^2,3x^2,3x^2,9x^3) .

If we want the integers to be distinct, then ( 1 , 2 , 3 , 6 ) (1,2,3,6) is the simplest solution. If ( a , b , c , d ) (a,b,c,d) is a solution, then so is ( a x 2 , b x 2 , c x 2 , d x 3 ) (ax^2,bx^2,cx^2,dx^3) , so there are infinitely many solutions with ( a , b , c , d ) (a,b,c,d) distinct.

If we're feeling really picky, we might want to insist that gcd ( a , b , c , d ) = 1 \gcd{(a,b,c,d)}=1 . There seem to be lots of solutions satisfying this additional criterion, but I haven't yet found a rule to generate them - any suggestions?

Steven Chase
Jul 15, 2019

Assuming we want non-zero integers, one example is ( a , b , c , d ) = ( 1 , 2 , 3 , 6 ) (a,b,c,d) = (1,2,3,6) . There are many others

Hana Wehbi
Jul 15, 2019

Is numerical example enough here? I can choose 0 3 + 1 3 + 2 3 = 3 2 0^3+1^3+2^3= 3^2 . Also, this solution is not unique.

Zero isn’t a positive integer

Richard Costen - 1 year, 11 months ago

I should edit the problem, sorry.

Hana Wehbi - 1 year, 11 months ago

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