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arXiv 0906.2848math.NTmath.CO
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三元二次型、模方程与某些正猜想

Ternary Quadratic Forms, Modular Equations and Certain Positivity Conjectures

Alexander Berkovich, William Jagy

摘要:

我们表明,许多拉马努金的模块化方程的程度3可以解释在积分三元二次型。这样我们就可以建立,对于N中的任何n,|{n= x(x+1)/2 + y^2 +z^2:x,y,z in Z}| >= |{n= x(x+1)/2 + 3y^2 + 3z^2:x,y,zinZ}|,只是提到这类许多类似的积极结果之一。特别地,我们证明了H. Yesilyurt和第一作者指出,对于N中的任何n,|{n= x(x+1)/2 + y^2 +z^2:x,y,z in Z}| >= |{n= x(x+1)/2 + 7y^2 + 7z^2:Z中的x,y,z}|.通过将判别式144,400,784,3600转化为相应的恒等式,我们证明了判别式为144,400,784,3600的某些三元形式的各种恒等式.在这个过程中,我们发现并证明了几个新的5次和7次模方程。对任意无平方数的奇数数S,其素因子分解为p_1. p_r,我们将S-亏格定义为三元二次型的2^r个特选亏格的并集,所有亏格的判别式都是16 S^2。在我们的研究过程中自然地产生了S-亏格的概念。这需要从判别式为-8 S的二元二次型的类注入到判别式为16 S ^2的三元二次型的类。

英文摘要:

We show that many of Ramanujan's modular equations of degree 3 can be interpreted in terms of integral ternary quadratic forms. This way we establish that for any n in N |{n= x(x+1)/2 + y^2 +z^2 : x,y,z in Z}| >= |{n= x(x+1)/2 + 3y^2 +3z^2: x,y,zin Z}|, just to mention one among many similar positive results of this type. In particular, we prove the recent conjecture of H. Yesilyurt and the first author stating that for any n in N |{n= x(x+1)/2 + y^2 +z^2 : x,y,z in Z}| >= |{n= x(x+1)/2 + 7y^2 + 7z^2: x,y,z in Z}|. We prove a variety of identities for certain ternary forms with discriminants 144,400, 784,3600 by converting these into identities for appropriate eta- quotients. In the process we discover and prove a few new modular equations of degree 5 and 7. For any square free odd integer S with prime factorization p_1.....p_r, we define the S-genus as a union of 2^r specially selected genera of ternary quadratic forms, all with discriminant 16 S^2. This notion of S-genus arises naturally in the course of our investigation. It entails an interesting injection from genera of binary quadratic forms with discriminant -8 S to genera of ternary quadratic forms with discriminant 16 S^2.

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