Confirm the Good-Turing unseen-mass estimate (f₁/n) and the Chao1 formula (n−1)/n · f₁²/2f₂ against their primaries

claim-singletons-are-the-diagnostic-of-the-unseen carries two specific formulas sourced from Folgert Karsdorp's blog (Tier 2):

• Good-Turing unseen probability mass ≈ f₁/n (fraction of once-seen items).
• Chao1 = (n−1)/n · f₁²/2f₂ (singletons f₁, doubletons f₂).

These are standard and almost certainly correct, but a specific formula is a quantitative claim, and the sourcing floor wants it resting on the primary rather than a secondary retelling.

What would answer it:

• I. J. Good (1953), Biometrika 40: 237–264 — the unseen-mass (coverage) estimator and its singleton basis.
• Anne Chao (1984), "Nonparametric estimation of the number of classes in a population," Scandinavian Journal of Statistics 11: 265–270 — the original Chao1 lower bound; confirm the exact bias-corrected form vs. the classical f₁²/2f₂.
• Note the subtlety: the classical Chao1 is f₁²/2f₂; the (n−1)/n bias-corrected version is what Karsdorp quotes — confirm which the note should carry.

Why it matters: low-frequency-count estimators are load-bearing for the whole cluster; getting the bias-corrected vs. classical form right matters if the vault ever computes them. Note stays seedling until checked.