Constraints for different probability measures to have the same expectation. - MathOverflow most recent 30 from http://mathoverflow.net 2013-06-19T22:13:58Z http://mathoverflow.net/feeds/question/37416 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/37416/constraints-for-different-probability-measures-to-have-the-same-expectation Constraints for different probability measures to have the same expectation. unknown (google) 2010-09-01T19:52:54Z 2010-09-01T19:52:54Z <p>Take different $D_i \in \mathbb{R} \rightarrow \mathbb{R}$ functions $f_1, f_2$ (i.e. $\exists x : f_1(x) \neq f_2(x)$). We have</p> <p>$E[f_1(x)] = E[f_2(x)]$</p> <p>Are there conditions that $f_1, f_2$ must satisfy for this to happen?</p> <p>I translated this problem to integral form as $\int_{E_1} x dg_1 = \int_{E_2} x dg_2$</p> <p>$g_1, g_2$ being the probability measures of $f_1(x)$ and $f_2(x)$, which can be easily calculated and $E_i$ the corresponding domains. Now, while the domains may be different, they are "similar", so we don't want to just fix domains conveniently -- instead, we want to study $f_1$ and $f_2$. Maybe there's a measure-theoretical backdoor into this, because every lead takes me to functional equations territory, which I can't handle at all. </p> <p>Disclaimer: Not a homework problem. So yes, I'll be profiting indirectly from the solution, even though it's a tiny piece to a large, mostly non-mathematical puzzle. Also, I hope I'm making myself clear and following the local etiquette. I'm not a native english person, and this is my first post on MO.</p>