The partition function is a statistical mechanics tool that sums over all possible states to determine thermodynamic properties like temperature.The partition function is like a catalog of tickets in an energy lottery: it lists all possible energy states of a system and indicates the chance of each occurring.
Boltzmann introduced the concept of statistical weight and entropy in the 1870s, but the explicit formula Z = Σ e^{-βE_i} for the canonical ensemble was introduced by Gibbs in 1902 in his book 'Elementary Principles in Statistical Mechanics'. Einstein and Bose in 1924 generalized it to quantum gases with integer spin, and Fermi and Dirac to half-integer spin, leading to Bose–Einstein and Fermi–Dirac statistics.
All thermodynamic quantities are expressed through Z: internal energy U = -∂ ln Z/∂β, heat capacity C_v = ∂U/∂T, pressure P = k_B T (∂ ln Z/∂V)_T. In the quantum case, Z = Tr(e^{-βĤ}), where Ĥ is the Hamiltonian. For non-interacting systems, it factorizes over single-particle states, but in general requires approximate methods.
Knowing just this sum, you can extract from it temperature, pressure, and even how a kettle on the stove will behave—everything that can be measured macroscopically.
| S_t | variable | CUSUM statistic at time t |
| S_{t-1} | variable | CUSUM statistic at previous time |
| z_t | variable | Observation or standardized value at time t |
| k | variable | Allowance or reference value (positive constant) |
| max | operator | Takes the maximum of two numbers — maximum |
| N | variable | column density — column density per square metre |
| S_ν | variable | line flux density — spectral flux density joule per square metre per second per he |
| Δν | variable | line width — frequency interval hertz |
| Q | variable | rotational partition function — partition function |
| T_rot | variable | rotational temperature — temperature kelvin |
| A_ul | variable | Einstein A coefficient — einstein a coefficient per second |
| Ω | variable | solid angle of source — solid angle steradian |
| h | variable | Planck constant — planck constant joule second |
| c | variable | speed of light — speed of light metre per second |
| g_u | variable | degeneracy of upper level — degeneracy |
| E_u | variable | energy of upper level — energy joule |
| k | variable | Boltzmann constant — boltzmann constant joule per kelvin |







