If you don't need provable correct answers then just change to RDF/CDF for
fast computation:
sage: c26 =
Graph('YsP@H?PC?O`?@@?_?G?@??CC?G??GG?E??@_??K???W???W???H???E_')
sage: sum(lam for lam in
c26.adjacency_matrix().change_ring(RDF).eigenvalues() if lam > 0)
19.870922489880787
On Friday, October 10, 2014 9:42:44 AM UTC+1, Nico Van Cleemput wrote:
>
> Today I came across the following problem. I start by computing the Gutman
> energy for a graph:
>
> sage:
> c26 = Graph('YsP@H?PC?O`?@@?_?G?@??CC?G??GG?E??@_??K???W???W???H???E_')
> sage:
> ge = sum(lam for lam in c26.adjacency_matrix().eigenvalues() if lam > 0)
> sage: ge
> 19.87092248988078?
>
> So far everything seems to be OK, but then I try
>
> sage: float(ge)
>
> Now you can wait for a very long time. (I actually didn't test to see how
> long it would exactly take) The type of ge is
> sage.rings.qqbar.AlgebraicNumber. Looking at the implementation of the
> float conversion, the problem seems to come from the following:
>
> sage: AA(ge)
>
> This indeed also seems to take forever. At that point I'm lost. I don't
> really understand much of that part of Sage, so I don't know how to look
> further for a solution to this problem. Is there another way to get a good
> float approximation of that value?
>
> Cheers
> Nico
>
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