Slide your cursor on the example to note the different elements
These sources have not been officially published, but may be cited by authors who attended a workshop. Example:[5] J.N. Mather. Arnold diffusion. Talk at the Workshop on Dynamical Systems. Oberwolfach. 15-21 July 2001.
You may try to locate an abstract or the contents of the talk by searching on the Internet. Alternatively you may also consult the bibliographical databases to see whether the cited author has published a paper on the subject - most probably - subsequent to the cited talk.
