Here, the charges flanking the closest interacting polycation-anion pair (at monomer s) also participate, via a product f P +,-|s-1 f P +,-|s f P +,-|s+1 . Here we use the notation that the 'nearest' monomer is s, and omit the index for the monomeric anion. If we assume no other participants, then we can write this product diagrammatically: