Bistromathics is the revolutionary new (and totally fictitious) field of mathematics in restaurants, as described by Douglas Adams in his book Life, the Universe and Everything:¹

 

Numbers written on restaurant bills within the confines of restaurants do not follow the same mathematical laws as numbers written on any other pieces of paper in any other parts of the Universe. This single statement took the scientific world by storm... So many mathematical conferences got held in such good restaurants that many of the finest minds of a generation died of obesity and heart failure and the science of math was put back by years.

 

Adams explains that just as Einstein found that space and time are not an absolute but depend on the observer's movement, so numbers are not absolute, but depend on the observer's movement in restaurants:

 

The first non-absolute number is the number of people for whom the table is reserved. This will vary during the course of the first three telephone calls to the restaurant, and then bear no apparent relation to the number of people who actually turn up, or to the number of people who subsequently join them after the show/match/ party/gig, or to the number of people who leave when they see who else has turned up. The second non-absolute number is the given time of arrival, which is now known to be one of the most bizarre of mathematical concepts, a recipriversexcluson, a number whose existence can only be defined as being anything other than itself. In other words, the given time of arrival is the one moment of time at which it is impossible that any member of the party will arrive... The third and most mysterious piece of nonabsoluteness of all lies in the relationship between the number of items on the bill, the cost of each item, the number of people at the table and what they are each prepared to pay for.

 

Reference

1. Adams, Douglas. Life, the Universe and Everything. New York: Harmony Books, 1982.