sequenceas

Sequenceas are a theoretical construct used to describe hierarchical or multi-layered sequence-like structures. Informally, a sequencea consists of an index that progresses along a base sequence and, at each position, an associated value or substructure that may itself be a sequencea. This recursive nature allows sequenceas to model data that is naturally nested or staged, such as streams of streams or sequences of parameterized computations.

Formally, a sequencea can be described as a family of sets or types (S0, S1, S2, …) indexed by natural numbers, together with a family of transition maps f_n: S_n → S_{n+1}. An infinite sequencea is the infinite chain S0 —f0→ S1 —f1→ S2 —f2→ …, while a finite sequencea terminates after a finite number of steps. If all S_n are singletons, a sequencea reduces to a conventional sequence; if the S_n carry richer structure, the sequencea encodes additional layers of information or computation.

Operations on sequenceas include projection (taking a sublevel of the index), reindexing (changing the progression of the base index), and concatenation (gluing two sequenceas by aligning their transitions). Morphisms between sequenceas are families of maps φ_n: S_n → T_n that commute with the respective transition maps, preserving the hierarchical progression.

Applications of sequenceas appear in areas such as modeling layered data streams, staged or compositional computation, and the semantic analysis of recursive or nested data structures. They provide a formal framework for reasoning about how complex sequences emerge from simpler, interconnected parts.

See also: sequences, streams, nested data structures, category theory, morphisms.