Prove Lemma 5.2.8

[js2357]

Prove Lemma 5.2.8.

Notes:

• I changed the statement of Lemma 5.2.8 to use ℝ instead of ℕ, and changed the exponent 9 * a - j to use subtraction in ℤ instead of ℕ. I think the lemma is only used to prove an inequality in ℝ≥0, so this shouldn't cause any problems.
• I corrected a minor error in the blueprint's proof, where it referred to u and u' as possible elements of B', rather than 𝒬 u and 𝒬 u' (and then needed an extra injectivity argument to finish the proof).

[fpvandoorn]

Thanks!