The representation-count formula for binary quadratic form numbers

Let a B{\mathcal B}-number be a positive integer represented by the binary quadratic form a2+ab+b2a^2+ab+b^2. Suppose that such a number is written in the general form

n=x23yaαbβcγ,n=x^2\cdot 3^y\cdot a^\alpha\cdot b^\beta\cdot c^\gamma\cdots,

where xx has no prime factor of the form 6k+16k+1, and a,b,c,a,b,c,\ldots are primes of the form 6k+16k+1. The exponents α,β,γ,\alpha,\beta,\gamma,\ldots are nonnegative integers. Representation-count formula. The number of distinct B{\mathcal B}-representations of nn, including the cases a=ba=b and either variable being zero, is

{12(1+(α+1)(β+1)(γ+1)),if all of α,β,γ, are even;12(α+1)(β+1)(γ+1),otherwise.\begin{cases} \frac{1}{2}\Bigl(1+(\alpha+1)(\beta+1)(\gamma+1)\cdots\Bigr), & \text{if all of }\alpha,\beta,\gamma,\ldots\text{ are even};\\ \frac{1}{2}(\alpha+1)(\beta+1)(\gamma+1)\cdots, & \text{otherwise.} \end{cases}

This gives an explicit divisor-exponent formula for the number of representations by a2+ab+b2a^2+ab+b^2, refining the preceding factorization results for B{\mathcal B}-numbers. The source does not provide evidence resolving the claim, so its status is left open.

Sources & referencesView supporting material

Primary source

Umesh P. Nair, “Elementary results on the binary quadratic form a^2+ab+b^2”, arXiv:math/0408107 (2004).

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