Consider the following premises. p_6 or ~u. p_6 -> u_1. u or ~u_1. Can we infer p_6 <-> u from them?
 From u or ~u_1 we can infer u_1 -> u. From p_6 -> u_1, u_1 -> u we can infer p_6 -> u. From p_6 or ~u we can infer u -> p_6. Finally, from p_6 -> u, u -> p_6 we can infer p_6 <-> u. Therefore, the answer is yes.