Before we end 2017......

Find the number of solutions for

a 2 + a b + b 2 = 2017 a^2 + ab + b^2 = 2017

where a Z a \in \mathbb{Z} and b Z b \in \mathbb{Z}

\infty 12 0 201 7 2 2017^2 1

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1 solution

Patrick Corn
Nov 22, 2017

Consider the equation over the ring R = Z [ ω ] , R = {\mathbb Z}[\omega], where ω \omega is a primitive third root of unity. It is an important fact that R R is a unique factorization domain; any element can be factored as a product of irreducibles, unique up to ordering and scaling by a unit. There is a norm function N : R Z N : R \to \mathbb Z defined by N ( a b ω ) = ( a b ω ) ( a b ω 2 ) = a 2 + a b + b 2 , N(a-b\omega) = (a-b\omega)(a-b\omega^2) = a^2+ab+b^2, and the nice thing about it is that it is multiplicative: N ( α β ) = N ( α ) N ( β ) . N(\alpha\beta) = N(\alpha)N(\beta).

Then we get N ( a b ω ) = ( a b ω ) ( a b ω 2 ) = 2017 , N(a-b\omega) = (a-b\omega)(a-b\omega^2) = 2017, and the prime 2017 2017 splits as the product of two irreducibles whose norm is 2017 2017 . (Anything whose norm is 2017 2017 must be irreducible because if it factored, the norm of one of the factors would have to be 1, so it would be a unit.)

By inspection, we can see N ( 41 7 ω ) = 4 1 2 + 7 41 + 7 2 = 2017 , N(41-7\omega) = 41^2 + 7 \cdot 41 + 7^2 = 2017, so there is at least one solution. (For primes p p not divisible by 3, a solution exists if and only if p 1 p \equiv 1 mod 3 , 3, by some basic algebraic number theory machinery.)

How many other solutions are there? Well, the factorization ( 41 7 ω ) ( 41 7 ω 2 ) = 2017 (41-7\omega)(41-7\omega^2) = 2017 is unique up to switching the factors and multiplying by a unit. There are exactly six units in Z [ ω ] , {\mathbb Z}[\omega], consisting of the sixth roots of unity generated by 1 + ω 1+\omega (this can be verified by fancy arguments involving Dirichlet's unit theorem, or pretty easily by direct computation). So there are a total of 2 6 = 12 2 \cdot 6 = \fbox{12} distinct solutions. For completeness, I'll list them here:

( ± 41 , ± 7 ) ( ± 48 , 41 ) ( ± 48 , 7 ) ( ± 7 , ± 41 ) ( ± 41 , 48 ) ( ± 7 , 48 ) (\pm 41, \pm 7) \\ (\pm 48, \mp 41) \\ (\pm 48, \mp 7) \\ (\pm 7, \pm 41) \\ (\pm 41, \mp 48) \\ (\pm 7, \mp 48)

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