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February 2012

4 participants
2 discussions

How to convert epsilon_tensor in a matrix?
by Marco André Ferreira Dias 26 Feb '12

26 Feb '12

Hi, I have a very naive question: my program compute some dirac_traces, and some epsilon_tensors arises in this process. I have to convert the tensorial structure to matricial to compute it numerically, but until now I don't figure out how to manipulate expressions like this ============= [[31],[0.0],[0.0],[-31]]~sigma*[[6.3],[0.0],[0.0],[6.30]]~alpha*[[19.33],[-1.108],[-5.4],[18.6]].gamma*eps.delta.sigma~gamma.alpha*[[16.781197650788506337],[4.96],[11.38],[11.2]]~gamma ============= Note that the episilon tensor is fully contracted by four (4x1) vectors. I will be glad for any suggestion given! Regards, Marco

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Simplifying a hierarchy of rational powers on positive symbols
by DerManu＠WorksLikeClockwork.com 21 Feb '12

21 Feb '12

Hi GiNaC Gurus, Consider the following expression: (2^(2/3)*x^2)^(3/2) where x is positive. I'd expect eval() to reduce this to 2*x^3, but it happens only, when calling evalf(). However, I'd like other possible parts of the expression to stay exact, so evalf() isn't an option. Here's the code for the above expression: possymbol x("x"); ex e = power( power(2,numeric(2)/numeric(3)) * power(x, 2) , numeric(3)/numeric(2) ); std::cout << e.eval() << " " << e.evalf() << std::endl; // I know eval is redundant, but it adds a dramatic element ;) Note this situation is somewhat related yet much simpler than Topic [1] because we're dealing with purely numeric powers here. Still it seems, this kind of simplification is just not wanted. Is that correct? Or is there a trick to make it all work easily? Would it be a sensible approach to write a map_function functor that grabs all powers that have positive muls as base and apply the power to all mul ops individually? Best regards, Emanuel [1] http://www.ginac.de/pipermail/ginac-list/2011-December/001862.html

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