Branching process extinction (critical)

Each individual has 00 offspring with probability 1/21/2 and 22 offspring with probability 1/21/2. Starting from one individual, what is the extinction probability?

Show hints (2)+
  1. Solve q=12+12q2q = \tfrac12 + \tfrac12 q^2 for the smallest root.
  2. Mean offspring exactly 11 is the critical case.

Answer

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1 - extinction is certain (critical mean = 1)

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