Opposite concepts revealing unified nature.

In optimization problems in applied mathematics, the duality gap is the difference between the primal and dual solutions.

In applied mathematics, weak duality is a concept in optimization which states that the duality gap is always greater than or equal to 0.

a class of symmetries in physics that link different string theories, theories which assume that the fundamental building blocks of the universe are strings instead of point particles.

(physics) The concept applying to all matter and radiation, but most evident in light and particles such as the electron, that properties of waves and of particles are exhibited simultaneously

Particles exhibit both wave-like properties.

In algebraic geometry, a branch of mathematics, Serre duality is a duality for the coherent sheaf cohomology of algebraic varieties, proved by Jean-Pierre Serre.