a^2 = b^3τ = √-t ( t > 0 ) : k( τ ) = 1/√5 k’( τ ) = 2/√5 ⇒ k( 2τ ) = ( √5-2 )^2 = 9-4√5 k’( τ/2 ) = ( 3-√5 )/2 ⇒ ( ( 3-√5 )/2 )^3 = 9-4√5 = ( √5-2 )^2 更に √5-2 = 2k( √-5 )k’( √-5 ) t = ?