program define pdf_chi2
    version 4.0

    local x = "`1'"
    local fx =  "`2'"
    local df = `3'

    quietly {
        local nub2=`df'/2
        local c=-`nub2'*log(2)-lngamma(`nub2')
        replace `fx'=exp(`c'-(`x'/2)+(`nub2'-1)*log(`x'))
    }

end