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Answer : There are actually two questions on MathOverflow that ask how to cite answers found on MathOverflow. I would recommend following their advice. One of them is a fairly old question, whose answer is somewhat outdated. Summarizing: Even though you may not be legally obligated to show where you got your answer (for example, when MO points you to a theorem already in the literature), it is a good idea — it never hurts to be generous and honest. (And if you're copying part of an answer, not citing it would be plagiarism.) There is a little button for each answer that says "cite" and will give you a citation. You might have to reformat this to make it fit in your journal's bibliography style. Perhaps the person who posted it actually found it elsewhere and can provide a reference to you. Alternately, if you are willing to be publicly known here, you can find a way to exchange emails and hence give proper credit to the individual(s). In the past, I'...

Answer : Ultimately there isn't a single valid approach here. The appropriate level of response depends on several factors, including the field in which the work is being performed, the nature of the conference, the status of the work relative to publishing practices in the field, and so on. For instance, if you're in a field where posters are peer-reviewed before a conference, then it's a much more significant issue than presenting a poster at, for instance, a Gordon Research Conference where posters are explicitly considered to be "for discussion only" and are not intended to be used for publication. As for the author order, some societies (for instance, the APS) expect that the poster (or talk) be given by the first author as listed at submission—whether or not this is fundamentally the way it would be listed in a paper. Looking at the ethical issue—has the work been published already somewhere or otherwise public? If so, your co-author may have thought ...