1. Quantization of four form fluxes and dynamical neutralization of the cosmological constant;Bousso;J. High Energy Phys.,2000
2. Some properties of O(32) superstrings;Witten;Phys. Lett. B,1984
3. Axions in string theory;Svrcek;J. High Energy Phys.,2006
4. String axiverse;Arvanitaki;Phys. Rev. D,2010