https://s0.wp.com/latex.php?latex=%5Cdisplaystyle+%7CS_n%28p%2C+a%29%7C+%3D+%5Cleft%28+p+-+%28-1%29%5E%7B+%5Cfrac%7Bp-1%7D%7B2%7D+%7D+%5Cright%29+%5Csum_%7Bb%3D0%7D%5E%7Bp-1%7D+%7CS_%7Bn-2%7D%28p%2C+b%29%7C+%2B+p+%28-1%29%5E%7B+%5Cfrac%7Bp-1%7D%7B2%7D+%7D+%7CS_%7Bn-2%7D%28p%2C+a%29%7C.&bg=ffffff&fg=333333&s=0&c=20201002