# Can a surface of the following type contain a line?

This is a follow-up question to my previous post: Sufficient conditions to tell whether a surface contains a line

Suppose that $f(x_1, x_2)$ is an irreducible binary form with integer coefficients of the variables $x_1, x_2$ over $\mathbb{Q}$ say, and suppose that $g(x_1, x_2)$ is another binary form with integer coefficients (not necessarily the same degree as $f$) that does not have a common component as $f$. In the case I am interested in this condition is satisfied vacuously since we assume $\deg(g) < \deg(f)$. Now, can the surface $$f(x_1, x_2) = g(x_3, x_4)$$ (in weighted projective space) contain any lines?

Note: the case $f = g = x^d + y^d$ has been treated by Heath-Brown, I believe.

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Unless I am mistaken, one cannot view what you have written down ($f(x_1,x_2)=g(x_3,x_4)$) as a being a surface unless $f$ and $g$ are homogeneous of the same degree, and also in which case you need to consider the corresponding variety as being projective. –  Daniel Loughran Nov 29 '11 at 23:25
You can consider it as a surface in a weighted projective space $\mathbb{P}(a,a,b,b)$ such that $a\text{deg}(f)$ equals $b\text{deg}(g)$ (and probably best to assume that $a$ and $b$ are relatively prime). –  Jason Starr Nov 30 '11 at 1:10
Yes; I do mean a surface in weighted projective space. –  Stanley Yao Xiao Nov 30 '11 at 14:58
But in weighted projective space what do you mean by a line? Just a rational curve? –  Felipe Voloch Nov 30 '11 at 15:14
Would the existence of a rational curve in a surface imply the existence of many rational solutions? –  Stanley Yao Xiao Dec 3 '11 at 4:18