I am trying to understand the notion of reduction of a problem to another problem. As it is known this has huge impact on classifying the complexity of a problem.
The definition of reduction involves the notion of instances. From wiki we read that a computational problem can be viewed as an infinite collection of instances together with a solution for every instance.
I do not really understand though what this actually means. If I consider the reduction problem of multiplication $$ a \times b = \frac{(a + b)^2 - a^2 - b^2}{2} $$ what would an instance be in this case? I am particularly interested in understanding what is an instance for a problem that cn be reduced to a SAT-3 problem.
One definition I ve found is that the instance of a problem is an exact specification of the data: for example, “The graph contains nodes 1, 2, 3, 4, 5, and 6, and edges (1, 2) with cost 10, (1, 3) with cost 14, …” if we have a problem containing a graph $G$. I still find this not quite a good definition.
Disclaimer: not a computer scientist here, just a string theorist.