... DUALITY1
This survey is dedicated to the memory of my father, Sr. Mariano García Salazar. 
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...ía-Compeán2
E-mail address: compean@fis.cinvestav.mx 
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... preserved3
Cosmological constant is generated by perturbation theory. Strings propagating in a ${\cal K}$ manifold in which all supersymmetries are broken distablizes the Minkowski vacuum. 
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