In the scientific journal style, one uses roman letters to denote elementary functions, whereas variables are written using italics.
ɪnðə saɪəntˈɪfɪk dʒˈɜːnəl stˈaɪl, wˈʌn jˈuːsᵻz ɹˈoʊmən lˈɛɾɚz tə dᵻnˈoʊt ˌɛlɪmˈɛntɚɹi fˈʌŋkʃənz, wˈɛɹæz vˈɛɹɪəbəlz ɑːɹ ɹˈɪʔn̩ jˈuːzɪŋ ɪtˈælɪks.