# Approximating common knowledge with common beliefs (Monderer and Samet, 1989)

I'm trying to understand Monderer and Samet's 1989 paper on approximate common knowledge.

I'm stuck in the last part of the proof of the "agreeing to disagree" Theorem A, where the upper bound of the posterior is established. The lower bound seems clear but I just cannot understand how one of the terms is written as (1-p). Am pasting a picture below of the part where I'm lost:

The last line which establishes the upper bound of r_i is unclear to me. Any help would be greatly appreciated. The link of the paper is :

https://ie.technion.ac.il/~dov/cpb_monderer_samet.pdf

(Theorem A is in page 180-181)

Let's rearrange the equation, \begin{align*} x &= r_i \frac{\mu\left( B^p_i\left( E \right) \right)}{\mu\left( E \right)} - \frac{\mu\left( X \cap \left[ B^p_i\left( E \right) \setminus E \right] \right)}{\mu\left( E \right)} \\ & \iff \underbrace{x \mu\left( E \right) + \mu\left( X \cap \left[ B^p_i\left( E \right) \setminus E \right] \right)}_{\text{LHS}} = r_i \mu\left( B^p_i\left( E \right) \right). \end{align*} Since all terms in $$\text{LHS}$$ are weakly positive, we can omit $$\mu( X \cap [ B^p_i( E ) \setminus E ] )$$ from $$\text{LHS}$$, giving $$x \mu\left( E \right) \leq r_i \mu\left( B^p_i\left( E \right) \right).$$ Since $$\mu( E ) \geq p \mu( B^p_i( E ) )$$, this directly implies $$x p \leq r_i$$.
Since $$\mu( E ) \leq \mu( B^p_i( E ) )$$ and $$x \geq 0$$, we have \begin{align*} &\frac{1}{\mu\left( E \right)} \left[ r_i \mu\left( B^p_i\left( E \right) \right) - \mu\left( X \cap \left[ B^p_i\left( E \right) \setminus E \right] \right) \right] \\ &\geq \frac{1}{\mu\left( B^p_i\left( E \right) \right)}\left[ r_i \mu\left( B^p_i\left( E \right) \right) - \mu\left( X \cap \left[ B^p_i\left( E \right) \setminus E \right] \right) \right] \\ &= \underbrace{r_i - \frac{\mu\left( X \cap \left[ B^p_i\left( E \right) \setminus E \right] \right)}{\mu\left( B^p_i\left( E \right) \right)}}_{\text{RHS}}. \end{align*} It must be that $$X \cap [ B^p_i( E ) \setminus E ] \subseteq B^p_i( E ) \setminus E$$, so $$\text{RHS} \geq r_i - \frac{\mu\left( B^p_i\left( E \right) \setminus E \right)}{\mu\left( B^p_i\left( E \right) \right)} = r_i - \frac{\mu\left( B^p_i\left( E \right) \right) - \mu\left( E \right)}{\mu\left( B^p_i\left( E \right) \right)} = r_i - \left( 1 - \mu\left( \left. E \right| B^p_i\left( E \right) \right) \right).$$ By definition (and this is inequality (9) in the text) $$\mu( E | B^p_i( E ) ) \geq p$$, thus $$\text{RHS} \geq r_i - \left( 1 - p \right).$$ Noting that the original equality was $$x = \text{RHS}$$, we have $$x \geq r_i - \left( 1 - p \right) \;\; \iff \;\; r_i \leq x + 1 - p.$$