If the manual uses a trick you missed (e.g., converting a sum to an integral using $\sum_p \approx \int dp$), write that trick down in your own notebook. You are now building your own "Rubinstein companion."

Before you attempt a rigorous derivation, try to guess the answer using scaling arguments. For example, if you are solving for the radius of gyration in a good solvent, write down the scaling law ($R \sim N^\nu$) first. If your rigorous derivation yields an exponent that contradicts the scaling law, you know immediately you made a mistake.

If the official Polymer Physics Rubinstein Solutions Manual remains inaccessible, do not despair. Several alternatives exist: