Relating invariant linear form and local epsilon factors via global   methods
  {We use the recent proof of Jacquet's conjecture due to Harris and Kudla, and the Burger-Sarnak principle to give a proof about the relationship between the existence of trilinear forms on representations of $GL_2(k_u)$ for a non-Archimedean local field $k_u$ and local epsilon factors which was earlier proved only in the odd residue characteristic by this author. The same method gives a global proof of a theorem of Saito and Tunnell about characters of $GL_2$ using a theorem of Waldspurger about period integrals for $GL_2$ and also an extension of the theorem of Saito-Tunnell by this author which was earlier proved only in odd residue characteristic.} 
Dipendra Prasad