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1
2R1.<x> = PolynomialRing(QQ)
3R2.<x,y> = PolynomialRing(QQ)
4
5def extended_field( f, A, B, deg_mult ):
6 # f: a polynomial whose roots are the x-coordinates of some points of an
7 # elliptic curve E: y^2 = x^3 + Ax + B.
8 # return value: a field containing the x and y coordinates of those points
9 # deg_mult: a positive integer known to be a multiple of the degree of the
10 # extended field.
11 #
12 # This function uses the properties of resultants (I can provide a pdf
13 # explaining how it works).
14 #
15 # It is useful to compute, e.g., the fields obtained by adjoining the
16 # coordinates of the n-division points of a point (using the n-uplication
17 # formulas to get the required polynomials).
18 #
19 # When used to compute the 2-division fields, it gives the same output as
20 # E.division_field(2).
21
22
23 g = y^2 - x^3 - A*x - B
24 res = f.resultant(g,x)
25 res = res.subs(y=x)
26
27 #K.<b> = f.splitting_field()
28 #print aux
29 print "+++ Computing splitting field of the following: +++"
30
31 pol = R1(res*f)
32 print pol
33
34 K.<b> = NumberField( R( pari(pol).nfsplitting(deg_mult) ) )
35
36 return K
37
38L = []
39
40for A in range(1,9):
41 for B in range(1,9):
42
43 print "Current list of examples:", len(L), "elements. List:"
44 print L
45
46 E = EllipticCurve([0,0,0,A,B])
47 print "*************************"
48 print E
49 print "of rank ", E.rank(), "with (some) points of infinite order:", E.gens()
50 print "CM:", E.has_cm()
51
52 rep = E.galois_representation()
53 print "mod 2 rep is surjective:", rep.is_surjective(2)
54
55 if E.rank() == 0:
56 print "Stopping because rank 0"
57 print ""
58 continue
59 if len(E.gens()) == 0:
60 print "Stopping because no points of infinite order found"
61 print ""
62 continue
63 if E.has_cm():
64 print "Stopping because CM"
65 print ""
66 print ""
67 continue
68 if not rep.is_surjective(2):
69 print "Stopping because mod 2 rep is not surjective"
70 print ""
71 continue
72
73 K_2.<a> = E.division_field(2)
74 print "2-division field:", K_2
75 P = E(0)
76 flag = False
77 for P in E.gens():
78 if len(P.division_points(2)) == 0:
79 flag = True
80 break
81 if not flag:
82 print "Stopping because the points found are 2-divisible"
83 print ""
84 continue
85 print "Taking the 2-division of P =", P
86
87 # The following polynomial is derived from the duplication formula
88 # (Silverman, p.54) using the x-coordinate of the 2-division point as
89 # an indeterminate.x
90 f_P = R1(x^4 - 4*P[0]*x^3 - 2*A*x^2 - 4*(2*B + A*P[0])*x + A^2 - 4*B*P[0])
91 #print f_P.roots()
92
93 M = extended_field( f_P, A, B, 24 ) # The 2-division field of P
94 print "2-division field of P:", M
95
96 if M.degree() != 24:
97 print "Stopping because 2-division of P is too small"
98 print ""
99 continue
100
101 div_pol_4 = E.division_polynomial(4)
102
103 #if div_pol_4.splitting_field('zz').degree() < 48:
104 # print "Stopping because splitting field of div_pol_4 < 48"
105 # exit()
106
107 K_4 = extended_field( div_pol_4, A, B, 96 )
108 print "4-division field:", K_4
109
110 if K_4.degree() != 96:
111 print "Stopping because mod 4 representation not surjective"
112 print ""
113 continue
114
115 if len(f_P.roots(ring=K_4)) == 0:
116 print "Stopping because M is not contained in K_4"
117 print ""
118 continue
119
120 print "----------------"
121 print "|Example Found!|"
122 print "----------------"
123 L.append((A,B))
124
125 print "*************************"
126 print ""

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