On 7/13/07, Soroosh Yazdani <[EMAIL PROTECTED]> wrote:
> Hi,
>
> I was writing a function to calculate the structure of cuspidal subgroup of
> J_0(N) without using modular symbols. I came across the following puzzling
> bug. Here is a procedure which produces a matrix
> def generate_cond5(l,r):
> M=Matrix(QQ,l,r)
> for j in range(l):
> divpoint=2**j
> for i in range(2**(l-1)):
>
> x=2*divpoint*ZZ(i).div(divpoint)+divpoint+ZZ(i).mod(divpoint)
> M[j,x]=1
> return M
>
> Now running "generate_cond5(3,8)" give the following output:
> sage: generate_cond5(3,8)
>
> [0 1 0 1 0 1 0 1]
> [0 0 1 1 0 0 1 1]
> [0 0 0 0 1 1 1 1]
>
> This is the correct output. However, if I change the second to last line in
> the procedure to "M[j,x]=1/2", the output matrix is the zero matrix.
> Similarly if I change the last line to "return M*(1/2)" I get the zero
> matrix. Any ideas what could cause this?
Yes. Python integers have confusing semantics (e.g., 1/2 == 0), and
probably the above
code isn't being pre-parsed. (Is it in a .py file?) A quick fix is to write
ZZ(1)/ZZ(2) or QQ('1/2') instead of 1/2.
william
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