In just over a year, I've written three posts for this blog about functionals, specifically about their application to probability theory [LINK], finding their stationary points [LINK], and the use of their stationary points in classical mechanics [LINK]. As a reminder, a functional is an object that maps a space of functions to a space of numbers. This got me thinking about what the reverse, namely an object that maps a space of numbers to a space of functions, looks like. To be clear, this is not the same as an ordinary function which, as an element in a space of functions, maps a space of numbers to a space of numbers.
As I thought about it more, I realized that this is a bit easier to understand and therefore more commonly encountered than a functional. An extremely glib way to describe such an object is a function of multiple variables. However, it may be more enlightening to describe this in further detail to avoid potentially deceptive images that may arise from that glib description.
In the discrete case, the matrix elements \( A_{ij} \) can be described as a map from integers to vectors, in which an integer \( j \) is associated with a vector whose elements indexed by an integer \( i \) are \( A_{ij} \). This is the essential idea behind seeing the columns of the matrix with elements \( A_{ij} \) as a collection of vectors. Formally, this maps \( i \to (j \to A_{ij}) \) where the map \( j \to A_{ij} \) defines a vector indexed by the free variable \( i \).
Similarly, in the continuous case, the function elements \( f(x, y) \) can be described as a map from numbers to functions, in which a number \( y \) is associated with a function whose
elements indexed by a number \( x \) are \( f(x, y) \). Formally, this maps \( x \to (y \to f(x, y)) \) where the map \( y \to f(x, y) \) defines a function indexed
by the free variable \( x \). These ideas are foundational to the development of more abstract notions of functions, like lambda calculus.
