TITLE: What is the purpose of the $\mp$ symbol in mathematical usage?
QUESTION [16 upvotes]: Occasionally I see the $\mp$ symbol, but I don't really know what it is for, except in conjunction with the $\pm$ symbol thus: $a \pm b \mp c$ which (I believe) means $a+b-c$  or  $a-b+c$ (please correct me if I am wrong). Is there any other mathematical usage for the $\mp$ symbol, particularly on its own ?

REPLY [5 votes]: You are correct; $\mp$ only makes sense in a formula that already has $\pm$.
One simple and useful example is that when $x$ is small, ${1\over{1\pm x}}\approx 1\mp x$.

REPLY [4 votes]: Like the other answerer, I've only seen it used in the same line as a $\pm$, to mean "positive when the other term is negative and negative when the other term is positive." So, for instance, if we were to say
$\pm a = \mp b$
that would imply that
$ a = -b $
and
$ -a = b $