I have been reading a online-book/blog/material on Quantum Mechanics, when I encountered a notation on a page and I have no idea what it means. See if you can help.
Here's the link and follows the paragraph where I am stuck.
Observe that exchanging either the incoming or the outgoing particles is tantamount to exchanging the two alternatives and, correspondingly, the two amplitudes, so that A2 takes the place of A1 and vice versa. Since the two amplitudes have the same magnitude, there is a complex number c of unit magnitude such that A2 = A1 c. In other words, multiplication by c = [1:β] represents an exchange of the incoming or outgoing particles.
If the incoming or outgoing particles are exchanged twice, then (i) A1 gets multiplied by $c^2$ and (ii) the original situation is restored. Thus A1 = A1 $c^2$, whence it follows that $c^2$ = [1:2β] = 1. This means that 2β must be equal to an integral multiple of 360°, and this leaves us with two possibilities: β = 0°, in which case A2 = A1, or β = 180°, in which case A2 = −A1.
I have put the notation in bold. What is it that the writer means exactly by $[1:\beta] \ \ and \ [1: 2 \beta]$ ?