How can I add reputation to one of the answer of my question in the math.stackexchange. Thanks
If you want to give additional points to somebody that answered your question you can place and award a bounty. To do this just click the link 'start a bounty' just below the question (yet after the comment section) and follow the instructions.
Note though that you cannot do this shortly after having asked the question. You need to wait for, I think, 48 hours.
For further information see the relevant section of the Help Center https://math.stackexchange.com/help/bounty