>>348
>>350
 a[n] = c[2^n],
 b[n] = c[2^n - 1],
ここに
 c[n] = {(1+√2)^n − (1-√2)^n}/√8,   (ビネ)
 c[n+1] = 2c[n] + c[n-1],      (線形漸化式)
 (c[n+1] - c[n])^2 - 2(c[n])^2 = (-1)^n,  (ペル)