Binary Operations for grouping

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Which of the following binary operations are closed?

  • subtraction of positive integers
  • division of nonzero integers
  • function composition of polynomials with real coefficients
  • multiplication of $2\times 2$ matrices with integer entries
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Let's start with the first one. Suppose that $a,b$ are each positive integers. Are we sure that $a-b$ is always a positive integer? In fact, no. For example, if $a=2$ and $b=10$, both positive integers, then $a-b$ is no longer a positive integer. Hence the set of positive integers is not closed under subtraction.