Use the variable name τ instead of t for Drinfeld modules

[xcaruso]

In the theory of Drinfeld module, the classical name of the noncommutative variable is τ. However, in Sage, we currently use t, which might conflict with other standard notations (often the variable name of the underlying function ring is denoted by t).
In this PR, we propose to shift to the notation τ. It just changes how the outputs are printed (by default), not the interface.

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

#40420, #40421

[mantepse]

Will this affect copy & paste, to reuse output?

[kryzar]

That's a fair question. In both the current state and the PR, the user has to instantiate a variable (t currently and τ in the PR).

τ is not on the keyboard, and in the docs we let tau be the variable (tau = phi.ore_variable()), but we suggest to do the following:

tau = τ = phi.ore_variable()

We acknowledge this is less convenient than the current state (the user has to do a copypasta), but t was a poorly named variable in the first place, as many people consider things such as $$t$$-motives...

[kryzar]

(@mantepse: we're not sure what your thumb-up means!)

[mantepse]

(@mantepse: we're not sure what your thumb-up means!)

It means that I am not against the proposed change.

[kryzar]

In that case, I looked through each job fail and the culprits seem to be:

• src/sage/libs/singular/function.pyx
• mpfi
• src/sage/rings/complex_arb.pyx

PR approved.

[user202729]

IPython tip: On a normal keyboard, you can type \tau<tab> to get τ.

If you're using some editor to edit a .sage file, yes you need something else.

p/s: maybe @kryzar want to review the new commit?

[xcaruso]

Great! Thanks for the tip!