 Reissner-Nordstrom trapped spheres

In ingoing Reissner-Nordstrom spacetime, orient $-{\partial }_r$ to the future and normalize the other radial null vector to ${\partial }_v+(f/2){\partial }_r$. The spherical null expansion in ingoing coordinates gives $-2/r$ and $f/r$. Hence the strictly future trapped surfaces among the round spheres occur exactly between the distinct horizons. At either horizon the outgoing null expansion vanishes. Inside the inner horizon the same round spheres are not future trapped.