Getting started using the software

This document will tell you about how to get started using atlas. To learn about how to launch atlas see Run atlas.

To quit atlas just type quit

(Or type ^Z to quit it momentarily and put it in the background. Then type fg to get it back in the foreground)

The atlas software will do basic arithmetic operations on integers, vectors, and matrices with integral or rational coefficients. These are the basic types of data that``atlas`` uses to store and give information on Lie groups structure and representations.

You can ask the software to compute basic operations on the different data types.

To set up a variable, for example x=1, type:

atlas> set x=1
Identifier x: int

The second line is atlas telling us that it recognizes x as an integer. atlas assigns a memory location in the computer for future operations involving the value assigned to that variable. We can verify we assigned the correct value to the variable by typing just x:

atlas> x
Value: 1

Sometimes we want to ask atlas to repeat the previous value or to use it to perform an operation on it. We use $ for this:

atlas> $
Value: 1
atlas> $+2
Value: 3

A useful way to change the value assigned to a variable is using the command :=. However, this works only as long as the new value is of the same data type as the old one. You may also notice in the example below that, if we don’t use this way of assigning a new value to the variable, we get a message “(overriding previous instance, with had type ….)”

There are other reasons for using this second method of redefining the value of a variable. They have to do with the software functionality

atlas> set x=2
Identifier x: int (overriding previous instance, which had type int)
atlas> x
Value: 2
atlas> x:=5
Value: 5
atlas> x:=2/3
Error during analysis of expression at <standard input>:33:0-6
Type error:
  Subexpression /(2,3) at <standard input>:33:3-6
  has wrong type: found rat while int was needed.
Expression analysis failed
Evaluation aborted.

Note that if you have not assigned a value to a variable you cannot use this method. You need to first use set:

atlas> z:=1
Error during analysis of expression at <standard input>:2:0-4
  Undefined identifier 'z' in assignment z:=1
Expression analysis failed
Evaluation aborted.
atlas> set z=1
Variable z: int
atlas> z
Value: 1
atlas> z:=2
Value: 2

The software will often accept a simpler data type (like an integer) in a place where a more complicated one (like a rational number) is required, as long as this can be done without ambiguity:

atlas> set y=3
Identifier y: int
atlas> set z=3/2
Identifier z: rat
atlas> y+z
Value: 9/2
atlas> whattype(y+z)
type: rat

This works in most cases. However, there are some exceptions when the software does not switch to the appropriate data type.

Another useful command is whattype:

atlas> whattype x
type: int

This tells us the data type (int) of the variable x. For more information on data types go to Basic Commands.

The file

When you launch atlas with the standard built-in commands only, you sometimes need other commands that are not included there, and you get an error telling you the command is undefined. The reason is that the command may be defined in a supplementary file that needs to be loaded. The filen contains most of the commands you will need to get familiar with the software.

We need to introduce two other commands.

< is the input command that will help you load the .at files that you want.

> is the output command. More on this later.

There are a few supplementary files that you may want to load right away as you launch the software. If you haven’t done so or you don’t remember, you can type < for example. If you get an error saying it failed to input the file, you need to quit atlas for a moment to learn how to do that.

The supplementary files are in the directory atlas-scripts. But atlas needs to know where they are. So you need to launch atlas again providing the path for these files. Go to Run atlas for information about this. (Make sure you are in the directory where you downloaded atlas and type ls to verify that the atlas-scripts directory is there).

You can also cd into that directory to see all the supplementary files available. These are all the .at files listed there.

After you launched the software with the path information, you can load the file

atlas> <
Starting to read from file 'atlas-scripts/'.

After this the software will list all the scripts included in the file that are now added to the built-in commands. You can see what you have now by scrolling back on your screen. As we cover more features of the subject we will need to load other .at files.

For more detail on the commands included in any .at file you can go to the file itself using more or emacs.