mfgaloisprojrep

Function: mfgaloisprojrep
Section: modular_forms
C-Name: mfgaloisprojrep
Prototype: GGp
Help: mfgaloisprojrep(mf,F): mf being an mf output by mfinit in weight 1,
 and F an eigenform, returns a polynomial defining the field fixed by the
 kernel of the projective representation associated to F.
Doc: \var{mf} being an \kbd{mf} output by \kbd{mfinit} in weight $1$,
 return a polynomial defining the field fixed by the kernel of the projective
 Artin representation attached to \var{F} (by Deligne--Serre).
 Currently only implemented for projective image~$A_4$ and~$S_4$.

 \bprog
 \\ A4 example
 ? mf = mfinit([4*31,1,Mod(87,124)],0);
 ? F = mfeigenbasis(mf)[1];
 ? mfgaloistype(mf,F)
 %3 = -12
 ? pol = mfgaloisprojrep(mf,F)
 %4 = x^12 + 68*x^10 + 4808*x^8 + ... + 4096
 ? G = galoisinit(pol); galoisidentify(G)
 %5 = [12,3] \\A4
 ? pol4 = polredbest(galoisfixedfield(G,G.gen[3], 1))
 %6 = x^4 + 7*x^2 - 2*x + 14
 ? polgalois(pol4)
 %7 = [12, 1, 1, "A4"]
 ? factor(nfdisc(pol4))
 %8 =
 [ 2 4]

 [31 2]

 \\ S4 example
 ? mf = mfinit([4*37,1,Mod(105,148)],0);
 ? F = mfeigenbasis(mf)[1];
 ? mfgaloistype(mf,F)
 %11 = -24
 ? pol = mfgaloisprojrep(mf,F)
 %12 = x^24 + 24*x^22 + 256*x^20 + ... + 255488256
 ? G = galoisinit(pol); galoisidentify(G)
 %13 = [24, 12] \\S4
 ? pol4 = polredbest(galoisfixedfield(G,G.gen[3..4], 1))
 %14 = x^4 - x^3 + 5*x^2 - 7*x + 12
 ? polgalois(pol4)
 %15 = [24, -1, 1, "S4"]
 ? factor(nfdisc(pol4))
 %16 =
 [ 2 2]

 [37 3]
 @eprog