Stefan–Boltzmann constant

https://dx.doi.org/10.1351/goldbook.S05964
Fundamental physical constant σ = 2 π 5 k 4 15 h 3 c 0 2 = 5.670 51 19 × 10 −8 W m −2 K −4.
Source:
CODATA Bull. 1986, 63, 1
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