I've find this symbol
$-\mathbb{R}_+^n$
of course we are talking of the set of nonnegative real numbers but what that minus sign means? For example I have
$X_i ∩ ( -\mathbb{R}_+^n)=${0}
what is the meaning of this intersection?
The context is theory of production, this expression is called "no free lunch" i.e. having production possibility set, $X_i⊂\mathbb{R}^n$, production is not free; there is no output without input consumption.