Index problem

edit

The line x = list(var('x_%i' % i) for i in (0..1)) defines x to be a list, and no matter what numbers you put in (m..n), lists in Python are indexed starting at 0. For example:

sage: x = list(var('x_%i' % i) for i in (1..2))                                           
sage: x                                                                                   
[x_1, x_2]
sage: x[0]                                                                                
x_1
sage: x[1]                                                                                
x_2
sage: x[2]                                                                                
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-7-481b12fa0bac> in <module>
----> 1 x[Integer(2)]

IndexError: list index out of range

That is, x[0] refers to the 0th element of the list x; the part of the command for i in (1..2) doesn't affect the list indexing. You could use a dictionary if you want different indexing:

sage: x = dict((i, var('x_%i' % i)) for i in (1..2))                                      
sage: x  # the numbers 1 and 2 are the keys: the allowable indices
{1: x_1, 2: x_2}
sage: x[1]                                                                                
x_1
sage: x[2]                                                                                
x_2