Abstract: Three-dimensional linear instability of flow past a circular cylinder is analyzed bymeans of a low-dimensional Galerkin method and Floquet instability theory. Some improvementson the construction of the azimuthal modes and calculation algorithm in the present analysisare made. The present results show that the ideal two-dimensional periodic flow around thecylinder is unstable with respect to a small spanwise disturbance. It has long-wavelength instabilityand the critical Reynolds number predicted is Re_c =1...