Let $S$ be a subset of the integers which is closed under multiplication. There are many possible choices of $S$:

- $S = \{-1, 1\}$.
- $S$ is the set of integers of the form $a^k$, where $a$ is fixed and $k \geq 0$ varies. For example, $S$ is the set of powers of 2.
- $S$ is the set of integers of the form $a^k$, where $a$ is varied and $k \geq 0$ is fixed. For example, $S$ is the set of all squares.
- $S$ is the set of integers divisible by some set of primes and not divisible by some other set of primes. For example, $S$ is the set of $B$-smooth integers, i.e. integers not divisible by any primes larger than $B$, or $S$ is the set of multiples of 13, or $S$ is the set of even numbers.
- Here is a more exotic choice: $S$ is the set of all integers which can be written as a sum of two squares (number theorists will recognize this as a special case of #4).

I have two questions: first, is there a simple reason why the collection of subsets of the integers which is closed under multiplication is so much richer than the collection of subsets of the integers which is closed under addition? Second, are there any other interesting choices of $S$ which aren't on my list?

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