Firstly, l define what I mean by structure equation. The solution of an algebraic equation is a number. The solution of a functional equation is a function, for instance, differential equations. The solution of a structure equation would be a structured set ,say, a geometry, a differentiable manifold, an algebra etc.
As far as I know, the only structure equation that has been widely investigated is the Einstein one, the solution of which is a differentiable manifold determined by its connection coefficients.
A learning system, modifies its own structure as he is learning new topics and handling new information. Thus, the unknown of a learning system theory is its structure sequence determined by the acquired information. I think that this topic must be investigated.