Mathematical Curiosities & Special Topics

Bertrand's Box Paradox Calculator

Compute the posterior chance that the other coin is gold after observing a gold coin.

Mathematical Curiosities & Special Topics

Explore the stated model

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Use the labelled probability protocol, game rule, finite bound, or numerical model. Simulations are seeded and reproducible; theoretical results remain separate. Inputs stay on this device.

Bertrand's box posterior

0.666666666667

P(other coin is gold | observed gold): 0.666666666667

Sampling protocol: Choose uniformly among GG, SS, and GS boxes, then choose one of its two coins uniformly and condition on observing gold

Equally likely observed-gold coins: 2 from GG, 1 from GS

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Quick guide

How to use this calculator

  1. Select the stated protocol, strategy, approximation, or conversion mode when one is available.
  2. Enter the labelled finite bound, probabilities, payoffs, sequence state, or special-function argument.
  3. Read the theoretical or deterministic result separately from any seeded empirical result.

Calculation method

Apply the stated mathematical model

Choose uniformly among GG, SS, and GS boxes, choose one of its two coins uniformly, and condition on gold; two eligible coins come from GG and one from GS, giving 2/3.

The calculator validates model hypotheses, finite work limits, probability domains, singular cases, and numerical approximation ranges before returning a result.

Worked example

Worked example

Under that sampling protocol, the posterior probability is 2/3.

Choose uniformly among GG, SS, and GS boxes, choose one of its two coins uniformly, and condition on gold; two eligible coins come from GG and one from GS, giving 2/3.

Supported inputs

Precision and limits

Model scope

Paradoxes depend on their information or randomization protocol. Queueing, games, fractals, and cellular automata use exactly the visible assumptions; changing those assumptions changes the answer.

Deterministic versus simulated

Exact combinatorial and theoretical probabilities are labelled separately from seeded pseudorandom simulations. Reusing a seed reproduces the same finite experiment.

Numerical methods

Finite double-precision arithmetic is used for probability models and special functions. Gamma uses a bounded Lanczos approximation, Bessel J uses a bounded convergent series, and erf/erfc disclose their approximation error.

Limits

Iteration, grid, candidate, trial, and series caps are visible in field labels or calculator-specific notes. A bounded trace never claims to prove an unresolved conjecture.

Calculator-specific rule

The 2/3 result depends on uniform box and coin selection before conditioning on the observed gold coin.