Published online by Cambridge University Press: 20 November 2018
Let  
 $G$  be the  
 $F$ -rational points of the symplectic group  
 $S{{p}_{2n}}$ , where  
 $F$  is a non-Archimedean local field of characteristic 0. Cogdell, Kim, Piatetski-Shapiro, and Shahidi constructed local Langlands functorial lifting from irreducible generic representations of  
 $G$  to irreducible representations of  
 $G{{L}_{2n+1}}\left( F \right)$ . Jiang and Soudry constructed the descent map from irreducible supercuspidal representations of  
 $G{{L}_{2n+1}}\left( F \right)$  to those of  
 $G$ , showing that the local Langlands functorial lifting from the irreducible supercuspidal generic representations is surjective. In this paper, based on above results, using the same descent method of studying  
 $S{{O}_{2n+1}}$  as Jiang and Soudry, we will show the rest of local Langlands functorial lifting is also surjective, and for any local Langlands parameter  
 $\phi \,\in \,\Phi \left( G \right)$ , we construct a representation  
 $\sigma $  such that  
 $\phi $  and  
 $\sigma $  have the same twisted local factors. As one application, we prove the  
 $G$ -case of a conjecture of Gross-Prasad and Rallis, that is, a local Langlands parameter  
 $\phi \,\in \,\Phi \left( G \right)$  is generic, i.e., the representation attached to  
 $\phi $  is generic, if and only if the adjoint  
 $L$ -function of  
 $\phi $  is holomorphic at  
 $s\,=\,1$ . As another application, we prove for each Arthur parameter  
 $\psi $ , and the corresponding local Langlands parameter  
 ${{\phi }_{\psi }}$ , the representation attached to  
 ${{\phi }_{\psi }}$  is generic if and only if  
 ${{\phi }_{\psi }}$  is tempered.