Do you have any information if the completely monotonic function of $n$-variables function has been defined yet?



Do you have any information if the completely monotonic function of n-variables has been defined yet?
You may like to look into [Ressel paper here](http://link.springer.com/article/10.1007/s11117-013-0244-6). If you don't have access then check out his [presentation slides](http://websites.math.leidenuniv.nl/positivity2013/presentations/ressel.pdf).


