A closed form for unitons

Unitons, i.e.\ harmonic spheres in a unitary group, correspond to \lq uniton bundles\rq, i.e.\ holomorphic bundles over the compactified tangent space to the complex line with certain triviality and other properties. In this paper, we use a monad representation similar to Donaldson's representation of instanton bundles to obtain a simple formula for the unitons. Using the monads, we show that real triviality for uniton bundles is automatic. We interpret the uniton number as the `length' of a jumping line of the bundle, and identify the uniton bundles which correspond to based maps into Grassmannians. We also show that energy-$3$ unitons are $1$-unitons, and give some examples.