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  • Chain Rule for Sequence Probability

Formula

Formula for an Impossible Initial Event

The formula for an initial event or token x0x_0x0​ in a sequence being an impossible event is: Pr⁡(x0)=0\Pr(x_0) = 0Pr(x0​)=0

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Updated 2026-07-02

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References


  • Reference of Foundations of Large Language Models Course

Tags

Ch.2 Generative Models - Foundations of Large Language Models

Foundations of Large Language Models

Foundations of Large Language Models Course

Computing Sciences

Related
  • Base Case for Sequence Probability

  • Joint Probability of a Generated Sequence using the Chain Rule

  • Relationship Between Joint, Conditional, and Marginal Log-Probabilities of Sequences

  • Derivation of Sequence Log-Probability via Chain Rule

  • Logarithmic Form of the Chain Rule for Sequence Probability

  • A language model is tasked with calculating the total probability of the three-token sequence 'the cat sat'. The model provides the following probability estimates:

    • The probability of the first token is Pr("the") = 0.1
    • The probability of the second token, given the first, is Pr("cat" | "the") = 0.5
    • The probability of the third token, given the first two, is Pr("sat" | "the", "cat") = 0.8

    Using the principle that the joint probability of a sequence is the product of the condit

  • Computational Stability of Sequence Probability

  • Which of the following expressions correctly decomposes the joint probability of a four-token sequence (x₁, x₂, x₃, x₄) using the chain rule of probability?

Learn After
  • Implication of an Impossible Initial Event

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  • A language model calculates the probability of a sequence of three tokens, {x₀, x₁, x₂}, using the formula: Pr(x₀, x₁, x₂) = Pr(x₀) * Pr(x₁|x₀) * Pr(x₂|x₀, x₁). If the model determines that the initial token, x₀, is an impossible event, what is the joint probability of the entire sequence?

  • Consequence of an Impossible Starting Token

  • A language model is calculating the probability of the sequence 'Zxq#w the cat sat'. If the model's vocabulary does not contain the token 'Zxq#w', making its initial probability zero, the model can still assign a non-zero probability to the entire sequence by considering the high probabilities of the subsequent words 'the', 'cat', and 'sat'.

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