This program was written to verify and expand some of the examples of mod 2 Galois representations from section
5.2 of Serre's paper "Sur les representations modulaires de degre 2 de Gal_Q." Serre refers to Mestre's paper
"La methode de graphes: Exemples et applications" for the details, but Mestre's exposition is incomplete and
contains some errors. We give a complete exposition and correct these errors, and we provide several new examples in
our notes on Serre's conjectures.
The main function of this program is "SerreInvariants" which takes as input a quintic, monic polynomial g with
integer coefficients, whose Galois group is A_5, and a positive integer N. Since A_5 = SL_2(F_4) = GL_2(F_4),
we get a mod 2 Galois representation (which is automatically odd) that turns out (for group theoretical reasons)
to be independent of the identification of the Galois group of G with GL_2(F_4). Our program returns the predicted
level, weight and character (=1) of a modular form giving rise to this representation, and lists the traces of Frob_l
for primes l < N up to Gal(F_4/F_2)-conjugacy (if l is ramified, the value given by our program should be ignored).
One can then consult William Stein's tables to try and find a corresponding eigenform of the right
level and weight (at least if the weight is bigger than 1).
We also include the program "MakeList" which takes an integer B as input and returns a list of those quintic polynomials
of the form
The code is documented, and the ideas underlying the algorithms we used are explained thoroughly in the aforementioned notes above.