Bistromathics, to put it simply, is a revolutionary new way of understanding numbers. Einstein realised that space is not absolute, but merely depended on the observers movement in space. Likewise, he realised time is not absolute but merely depended on observers movements in time. Bistromathics, in the most basic sense, is the realisation that numbers are not absolute, but merely depend on the observers 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 nonabsolute 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.
Recipriversexclusons now play a vital role in many mathematical branches, including statistics and accountancy and also form the basic equations used to engineer the Somebody Else's Problem field.
The third and most mysterious piece of non-absoluteness 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. (The number of people who have actually brought any money is only a sub-phenomenon in this field.)
The discrepancies at this point, had never been investigated thoroughly because nobody took them seriously. The startling truth is, 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.
That truth revolutionised the scientific world completely. Mathematical conferences were held in fantastic restaurants, and many of the generations finest minds died of obesity and heart failure. This upset put the science of mathematics back years.
Bistromathics is also what the entire 5th chapter of Douglas Adams' Life, The Universe And Everything is dedicated to describing.