F. Bastianelli et N. Ohta, BRST OPERATOR FOR THE LARGE N=4 SUPERCONFORMAL ALGEBRA, Physical review. D. Particles and fields, 50(6), 1994, pp. 4051-4059
We review the detailed structure of the large N=4 superconformal algeb
ra and construct its BRST operator which constitutes the main object f
or analyzing N=4 strings. We then derive the general condition for the
nilpotency of the BRST operator and show that there exists a line of
critical N=4 string theories.