and closed string sectors are combined the Yang-Mills coupling from the open string sector has g 2 Y M = e $^{Φ}$. All of the fields listed above are NS-NS fields (Neveu-Schwarz-Neveu- Schwarz). R-R (Ramond-Ramond), NS-R and R-NS fields also exist. The NS-R and R-NS fields are not a concern to us - they are simply the fermionic partners of the bosonic sector. However the R-R fields are sourced by D- branes [9] and so are also present in the spectrum. They enter as field strengths of antisymmetric fields as shown in (1.46) and (1.47). For the AdS/CFT correspondence applied to 3 + 1-dimensional field theories, ten-dimensional type IIB string theory is of central importance, and in particular its low-energy limit where strings become point-like and string theory becomes supergravity. There exists no completely satisfactory action for the type IIB supergravity, since it involves an antisymmetric field C$_{4}$ with self-dual field strength F$_{5}$ . However, it is possible to write an action involving both dualities of C$_{4}$ , and then impose the self-duality as a supplementary field equation. In this way one obtains (see for example [42–44]) 29 S$_{IIB}$ = 1 4 κ 2 B ∫ √$_{Ge}$$_{−}$ $^{2Φ}$(2 R$_{G}$ + 8 ∂$_{µ}$ Φ ∂ $^{µ}$Φ − | H$_{3}$ | $^{2}$) (1.46) − 1 4 κ 2 B [ √$_{G}$$_{(}$ | F$_{1}$ | 2 + | ˜ F$_{3}$ | 2 + 1 2 | ˜ F$_{5}$ | $^{2}$) + C$_{4}$ ∧ H$_{3}$ ∧ F$_{3}$ ] + fermions , • B , known as the Kalb-Ramond or NS-NS B-field, is the generaliza- tion of the one-form potential A$_{µ}$ of electrodynamics. For point-like particles moving in an electromagnetic potential the action is given by ∫ A$_{µ}$dx $^{µ}$. For closed strings moving in a Kalb-Ramond potential, the action is given by ∫ B$_{µν}$dx $^{µ}$dx$^{ν}$.