I have asked a question that I haven't get a full answer http://math.stackexchange.com/questions/15075/do-we-have-a-proof-of-the-infiniteness how can i give motivation in order to take one?
Please refer to the FAQ. In the section "What if I don't get a good answer" it is explained that the offering of a bounty (so that the question goes into the Featured tab) requires the person offering the bounty to have at least 75 reputation points. Part of it is because the minimum bounty you can offer is 50 points, and the bounty comes straight out of the reputation points of the person offering it.
So currently you are not eligible to offer a bounty. (But you will soon, if you participate in the site and get more up-votes.)
Please refer to the FAQ for more details about the bounty system and how to use it.
Although this is not quite the right place to say this: as I noted in my answer to your question, it is likely the case that answering your question precisely will require techniques on the cutting edge of what is currently known.
Given this, it doesn't seem so likely to me that an expert is going to wander along and answer your question. Without wanting to be too discouraging, I think it might be the case that you've gotten all the feedback you are likely to get form a Q&A forum like this. If you want to pursue your question further, you may have to learn the relevant techniques yourself and see what you can do.