= HyRec: A fast and highly accurate primordial hydrogen and helium recombination code by Yacine Ali-Haïmoud and Christopher M. Hirata
{c}
{tag=Work by Yacine Ali-Haïmoud}
{tag=Work by Christopher M. Hirata}
{title2=2011 journal publication; 2010 preprint}
{title2=10.1103/PhysRevD.83.043513}
Physical Review D 83, 043513. Section VI.B, equation (97), explicitly gives the factor four from neutral-helium spin degeneracy; the following section treats departure from Saha equilibrium.
https://arxiv.org/abs/1011.3758
https://arxiv.org/html/1011.3758v2
https://doi.org/10.1103/PhysRevD.83.043513
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