[susam]

Even in modern mathematics, the postfix notation is used in some places. One example I can recall is permutations.

Consider the following permutation:

  f = (1 2 3 4)
      (1 4 2 3)

This is typically written in cycle-notation like this:

  f = (1)(243)

If we look at the cycle notation, f maps each symbol to the one on the right. For example, it maps 1 to 1, 2 to 4, 4 to 3, and 3 to 2.

Now if we want to find out what happens to the sequence (4 3 2 1) when we apply f to it, we normally write it as

  (3 4 2 1)f

The argument comes first. The function name comes next. It makes it quite convenient to work with permutations, composition of permutations, etc. For example, the result for the above application is:

  (3 4 2 1)f = (3 4 2 1)(1)(243) = (2 3 4 1)

[wging]

You see it in some treatments of abstract algebra too. (Which are not unrelated to permutations in any case.) I think at least one book used in my undergrad used postfixes for function application, but I can't remember which.

[twnettytwo]

Was it perhaps 'A course on Group Theory' by John S. Rose?

[wging]

Definitely not. (He may do that, I just hadn't ever heard of that book before today.) I think it was one of Isaacs, Herstein, or Artin, but I'm a bit too lazy to dig through my old books at the moment.

[JadeNB]

> Definitely not. (He may do that, I just hadn't ever heard of that book before today.) I think it was one of Isaacs, Herstein, or Artin, but I'm a bit too lazy to dig through my old books at the moment.

Herstein does it.

[wging]

Ah, thanks!