...R

R₂₃ = ([O  II]$\lambda$3727+[O  III] $\lambda\lambda$4959,5007)/H$\beta$.

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...P

P = [O  III] $\lambda\lambda$4959,5007/([O  II]$\lambda$3727+[O  III] $\lambda\lambda$4959,5007).

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...2

N2 = log ([N  II]$\lambda$6583/H$\alpha$).

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... O3N2

O3N2 = log (([O  III]$\lambda$5007/H$\beta$)/([N  II]$\lambda$6583/H$\alpha$)).

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... Ar3O3

Ar3O3 = [Ar  III] $\lambda7135$/[O  III] $\lambda 5007$.

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... S3O3

S3O3 = [S  III] $\lambda9069$/[O  III] $\lambda 5007$.

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... MPA/JHU group

See http://www.mpa-garching.mpg.de/SDSS/

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