There are 18 (and possibly 19) integers that are not of the form $ xy + yz + xz $ with positive integers $x, y, z$. The same 18 integers appear as exceptional discriminants for which no indecomposable positive definite binary quadratic form exists. We show that the two problems are equivalent.
@article{1103749835,
author = {Peters, Meinhard},
title = {The Diophantine Equation $ xy + yz + xz = n $ and Indecomposable Binary Quadratic Forms},
journal = {Experiment. Math.},
volume = {13},
number = {1},
year = {2004},
pages = { 273-274},
language = {en},
url = {http://dml.mathdoc.fr/item/1103749835}
}
Peters, Meinhard. The Diophantine Equation $ xy + yz + xz = n $ and Indecomposable Binary Quadratic Forms. Experiment. Math., Tome 13 (2004) no. 1, pp. 273-274. http://gdmltest.u-ga.fr/item/1103749835/