Changeset 7674:33cb0113c435

Timestamp:

11/29/07 13:58:59 (5 years ago)

Author:

Robert Bradshaw <robertwb@…>

Branch:

default

Message:

Fix bug in char(k)=2 elliptic curve random element.

The code assumed that given an x, there were always exactly 2 possible y-values.

File:

• sage/schemes/elliptic_curves/ell_finite_field.py (modified) (2 diffs)

sage/schemes/elliptic_curves/ell_finite_field.py

@@ -217 +217 @@
                 f = y**2 + a1*x*y + a3*y - x**3 + a2*x**2 + a4*x + a6
                 roots = f.roots()
-                if roots == []:
-                    continue
-                if random.random() < 0.5:
-                    y = roots[0][0]
-                else:
-                    y = roots[1][0]
-                return self([x,y])
+                n = len(roots)
+                if n:
+                    y = roots[random.randint(0,n-1)][0]
+                    return self([x,y])
         else:
             while True:
@@ -230 +227 @@
                 try:
                     m = d.sqrt(extend=False)
+                    if random.random() < 0.5:
+                        m = -m
                     y = (-(a1*x + a3) + m) / k(2)
                     return self([x,y])