An analytical study on the existence of solitary wave and double layer solution of the well-known energy integral at M= Mc
A general theory for the existence of solitary wave and double layer at M= Mc has been discussed, where Mc is the lower bound of the Mach number M, i.e., solitary wave and/or double layer solutions of the well-known energy integral start to exist for M> Mc. Ten important theorems have been proved to confirm the existence of solitary wave and double layer at M = Mc. If V(ϕ)({\equiv}V(M,ϕ)) denotes the Sagdeev potential with ϕ is the perturbed field or perturbed dependent variable associated with the specific problem, V(M,ϕ) is well defined as a real number for all M {\in} \mathcal{M} and for all ϕ {\in} Φ, and V(M,0)=V'(M,0)=V"(Mc,0)=0, V"'(Mc,0)<0 (V"'(Mc,0)>0), \deltaV/\deltaM < 0 for all M({\in} \mathcal{M}) > 0 and for all ϕ({\in} Φ) > 0 (ϕ({\in}Φ) < 0), where " '{\equiv} δ/δϕ ", the main analytical results for the existence of solitary wave and double layer solution of the energy integral at M= Mc are as follows. Result-1: If there exists at least one value M0 of M such that the system supports positive (negative) potential solitary waves for all Mc Mc, then there does not exist positive (negative) potential solitary wave at M= Mc. Result-3: It is not possible to have coexistence of both positive and negative potential solitary structures (including double layers) at M= Mc. Apart from the conditions of Result-1, the double layer solution at M= Mc is possible only when there exists a double layer solution in any right neighborhood of Mc. Finally these analytical results have been applied to a specific problem on dust acoustic waves in nonthermal plasma in search of new results.