Fix index offset in find_point call for Schottky class

Author: mmasdeu

darmonpoints/schottky.py

@@ -802,7 +802,7 @@ def _u_function(self, i, prec, a, naive=False):
         """
         gamma = self._generators[i]
         if a is None:
-            a = self.find_point(gamma, idx=i) # self.a_point()
+            a = self.find_point(gamma, idx=i+1) # self.a_point()
             K = a.parent()
         else:
             a = self.base_ring()(1) * a