Consider the following premises. p_13 <-> r_2. p_13 <-> u. u -> r_2. Can we infer r_2 <-> p_13 from them? If possible, name the inference rules being used at each step.
 From p_13 <-> u we can infer p_13 -> u via biconditional elimination. From p_13 -> u, u -> r_2 we can infer p_13 -> r_2 via transitivity. From p_13 <-> r_2 we can infer r_2 -> p_13 via biconditional elimination. Finally, from r_2 -> p_13, p_13 -> r_2 we can infer r_2 <-> p_13 via biconditional introduction. Therefore, the answer is yes.