Mathematics makes a nice distinction between the usually synonymous terms elementary and simple, with elementary taken to mean that not much mathematical knowledge is needed to read the material and simple to mean that not much mathematical ability is needed to understand it.
Julian Havil, 2012.
When the mathematically different meanings of these two adjectives collapse into one, the adjective complex makes for a suitable antonym. Julian Havil discusses the distinct meanings of the adjectives elementary and simple within mathematical context. He also provides an example in which their distinct meanings become difficult to grasp or where both words even regain identical meaning: the proof that ζ(3) is an irrational number, which has been achieved in quite different ways by (1) Roger Apéry, (2) Frits Beukers, (3) Wadim Zudilin, and others. An interesting, yet complex topic—even when illustrated in simple terms.
Keywords: semantics, adjectives, antonym, word disambiguation.
Julian Havil: The Irrationals. Princeton University Press, Princeton and Oxford, California, 2012; pp. 152-153.