If sentences like
I have something to eat.
are translated as
Mi havas ion por manĝi.
as it would be in French, then what about sentences like
I have something about which to think.
I have nothing onto which to jump.
That is to say, is the "por" still necessary here?
Mi havas ion, pri kio [por] pensi.
Mi havas nenion, sur kion [por] salti.
No, you don't need the por anymore, nor would it even be right to add it in, in my opinion.
Mi havas ion, pri kio pensi.
Mi havas nenion, sur kion salti.
Of course, you can also rephrase if you prefer:
Mi havas ion, pri kio mi povas pensi.
Mi havas nenion, sur kion mi povas salti.
The English sentences here are somewhat misleading, I think, because the to in the last two examples I think is different from the to in the first example. In the first example, it's a particle meaning something like 'in order to'; in the later examples, it's just a particle introducing the infinitive. Given that, you can see that English is actually not that different here, either.
