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Foundations of Data Science

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Since E(z 1 |extinction) = f ′ (q), E (z 2 |extinction) = (f ′ (q)) 2 . Similarly, E (z i |extinction) =<br />

(f ′ (q)) i . The expected size <strong>of</strong> the tree is the sum <strong>of</strong> the expected sizes <strong>of</strong> each generation.<br />

That is,<br />

∞<br />

Expected size <strong>of</strong><br />

tree given extinction = ∑<br />

E (z i |extinction) =<br />

i=0<br />

∞∑<br />

(f ′ (q)) i =<br />

i=0<br />

1<br />

1 − f ′ (q) .<br />

Thus, the expected size <strong>of</strong> an extinct family is finite since f ′ (q) < 1 provided m ≠ 1.<br />

The fact that f ′ (q) < 1 is illustrated in Figure 4.10. If m 1, then q ∈ [0, 1) and again f ′ (q)

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