What I Learned About Mathematics Today: Characteristics of Positive Integers

Any two positive positive integers sum to a positive integer and multiply to a positive integer.  This is being “closed” to addition and multiplication.

Though there is a finite number of integers that sum to, or are multiples of, a positive integer, the number that subtract to, or are divisors of, is infinite(!)

[courtesy of “A Year of Mathematics” calendar by Steven J. Alexander]