差等 is read as さとう (satou) and means gradation.
さとう
satou
とうさきゅうすう
arithmetic series
さべつ
discrimination (against people)
さ
difference
さしひき
ebb and flow
さしあげる
to do (for someone)
さしだす
to present
さしつかえ
hindrance
さしつかえる
to interfere (with)
さす
to be tinged with
さしかかる
to come near (to)
さしこむ
to have a griping pain
さしとめ
prohibition