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Could it be....?
I have been doing some symbolic matrix computations in which the product of a bunch of square matrices may or may not have all rows equal. To avoid a very slow FullSimplify of the final product (this program has to run for hundreds of hours) to see if all the rows are equal, I gave the only variable in the computation a numeric value after the product was computed. In other words, the way I wrote it, the matrix products are done symbolically but the final product is tested numerically. I observe some odd behavior in the timing of the matrix multiplies. Is it possible that Mathematica, after noting that the final product is used only for numeric evaluation, decides on its own to do the multiplies numerically to speed things up? If so, is it possible that it decides this only after the code has been run a few times? Sounds nuts, maybe, but this would explain a lot. (When I look at the matrix products, they are symbolic.) I would love to solve this mystery. Thanks for any information. Steve