Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† ărithmētĭcus, a, um, adj., = ἀριθμητικός, of or pertaining to arithmetic, arithmetical: ratio, Vitr. 10, 16.
Subst.: ărithmētĭca, ōrum, n., arithmetic: in arithmeticis satis exercitatus, Cic. Att. 14, 12 fin.