I bought a great collection of shorts by a specific author a few years ago, lent it out, and have not seen it since. One of my favorite stories was about an alien who could only consume alcohol. Once the alien tried it, it was declared that earth had the best food in the galaxy. The alien also explained that people could transcend to "higher" forms of existence by reaching a level of morality of 1. After several generations, the world started to act right and were constantly reminded by other species they could evolve, especially since aliens constantly visited earth to eat the finest food in the galaxy. Hope someone knows what I'm referring to. Thanks for the help.
I don't know about the math themed collection, but the story you describe certainly sounds like Joe Haldeman's All The Universe in a Mason Jar.
For one thing, it is about an alien who can only consume alcohol.
When it is served alcohol from a famous moonshiner, it states "I do not lie. This is the best food I have ever tasted."
After a rather grim period of transition, the denizens of Earth settled down to concentrating on being good, trying to reach Class 3, the magic level.
It would take surprisingly few generations. Because humankind had a constant reminder of the heaven on Earth that awaited them, as ship after ship drifted down from the sky to settle by a still outside a little farm near New Homestead, Florida: for several races, the gourmet center of Sirius Sector.
There is some mathematical content in the story, when the retired-math-professor hero is trying to communicate with the alien.