FFT
[FLleft(x
ight)=a_0+a_2 imes x^2+cdots+a_{n-2} imes x^{n-2}
]
[FRleft(x
ight)=a_1 imes x+a_3 imes x^3+cdots+a_{n-1} imes x^{n-1}
]
[Fleft(x
ight)=FLleft(x^2
ight)+x imes FRleft(x^2
ight)
]
[Fleft(omega_n^k
ight)=FLleft(omega_{n/2}^k
ight)+omega_n^k imes FRleft(omega_{n/2}^k
ight)
]
[Fleft(omega_n^{k+n/2}
ight)=FLleft(omega_{n/2}^k
ight)-omega_n^k imes FRleft(omega_{n/2}^k
ight)
]
(y) 点值，(a) 系数。
[c_k=sum_{i=0}^{n-1}y_ileft(omega_n^{-k}
ight)^i
]
[a_k=dfrac{c_k}{n}
]