Fusing E-strings to Heterotic Strings:Haghighat · Physical review. D/Physical review. D.2014PDF1.0 MB
Closed Strings from Open Bosonic StringsSiopsis · Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D. Particles and fields1989PDF287 KB
Coding Strings by Pairs of StringsChung · Society for Industrial and Applied Mathematics1985PDF1.3 MB
Locally Flat Strings and Half-StringsLacher · Proceedings of the American Mathematical Society1967PDF209 KB