reference 
[1] A kernel two sample test 
[2] Optimal kernel choice for large-scale two-sample tests 
[3] Deep domain confusion: maximizing for domain invariance 
[4] Learning transferable feature with deep adaptation nets 
[5] Deep transfer network：Unsupervised domain adaptation 
[6] Adaptive visual category models to new domains 
[7] Geodesic flow kernel for unsupervised domain adaptation 
[8] Transfer sparse coding for robust image representation