Does anyone know the name/author of the model theory paper where it's proven that you can define the $ < $ relation on $ (\mathbb{Q}, +, \cdot, 1, 0) $?



"Less than" formula for complete theory of the rationals
Every natural number is the [sum of four squares](http://en.wikipedia.org/wiki/Lagrange%27s_four-square_theorem), and so you
can define the positive rational numbers as those of the
form $(a^2+b^2+c^2+d^2)/(e^2+f^2+g^2+h^2)$, where the denominator is not zero, and this is
expressible in your language. And the order is defined by
$x\lt y\iff y-x$ is positive.


