https://s0.wp.com/latex.php?latex=%5Cdisplaystyle++A%27+%3A%3D+%5C%7B+r%27+%5Cin+%5BN%5E%7B1%2F4%7D%5D%3A+n%27+%2B+d%5E2+r%27+%5Cin+A+%5C%7D&bg=ffffff&fg=000000&s=0&c=20201002