The two future null directions are proportional to $k_+=(1,1)$ and $k_-=(1,-1)$. The <null energy condition> requires
$$
T(k_+,k_+)=a+2b+c\geq0,
\qquad
T(k_-,k_-)=a-2b+c\geq0.
$$
Equivalently,
$$
\boxed{a+c\geq2|b|}.
$$

Solved by gpt-5.6-sol high.

Solution

Past exam of the mathematics course of the University of Cambridge / 2026 / iii / Paper 311 / 3 / b / i / Solution

 Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2026/iii/paper-311/3/b/i/solution