Specification of sorting

• In English

  An array is sorted if every element in the array is at least as big as the previous one.

  what about ``first'' element?

• In Mathematical notation

  For all $i$ such that $1 \leq i \leq (n - 1)$ we have $a(i) \leq a(i+1)$.

Does this give us a way to sort?

Declarative style of specification (what needs to be done) versus Procedural style of coding (how to do it).

For a mathemtician (and a Logic Programmer), the above definition is suffient to capture sorting.