Variations on a theme: the cohomology of the Steenrod algebra, providing the $E_2$ page of the Adams spectral sequence.

This program computes $\mathrm{Ext}_A(M,\mathbb{F}_p)$, where

- $A$ is the Steenrod algebra or its subalgebra A(n),
- $M$ is an A-module, either one of a few standard examples, or else specified in Bruner’s MDF format,
- $p$ is any prime.

A couple of other miscellaneous computations are included, such as the $E_2$ page of the Cartan–Eilenberg / algebraic Novikov spectral sequence.

Output takes the form of a scrollable chart, with products by the first three Hopf elements drawn. For a laugh, there’s also a 3D viewer for the trigraded computations.

This software is open-source, hosted in this github repository.

# Download & use

Running the program requires only one JAR file, available here.

For trigraded computations in the 2D viewer, there are controls to limit the visible range for the third grading; these are also jointly controlled by the `PgUp`

and `PgDn`

keys on your keyboard, in case you want to quickly step through cross-sections.

For trigraded computations in the 3D viewer, you can `drag`

the mouse to rotate, `Shift+drag`

to pan the view, and either `scroll`

or `Ctrl+drag`

to zoom.

If there are features you’d like to see in this program, please get in touch!

# Module format

The program can load modules specified in the ‘module definition format’ of Bob Bruner.

The format changes slightly for odd primes, since we need to include coefficients in the action. When working with odd primes, the format for the action lines should be:

```
g r k c_1 g_1 c_2 g_2 ... c_k g_k
```

to indicate that Choice of integer representative (mod p) of the coefficients doesn’t matter.

So e.g. $S/p$ as a module over the mod-p Steenrod algebra is given as:

```
2
0 1
0 1 1 1 1
```

for any odd p. (Usually, of course, the same module file won’t work for all odd primes.)

# Compiling

The source includes shell scripts `make`

for compiling, `run`

for running, and `prof`

for profiling. Currently they’re a bit specialized to my machine, in particular referring to a Java 6 runtime `rt.jar`

in a subfolder, but it shouldn’t be too hard to get it built if you’re into Java. Feel free to contact me.

# Acknowledgements

Most of the features in this program were developed during a period of collaboration with Michael Andrews, and so he has guided its direction to a large extent. Thanks also to Mark Behrens for feature suggestions.