2.3.1

The function $$2^x$$ (or $$e^x$$ or any $$a^x$$ for $$a>0$$) sends $$\mathbb{R}$$ to $$(0, \infty)$$ and satisfies $$2^{x+y} = 2^x 2^y$$. The inverse function is the logarithm. These make a bijection.