ζ⁡(s,a)=12⁢a-s+a1-ss-1+2⁢∫0∞sin⁡(s⁢arctan⁡(x/a))(a2+x2)s/2⁢(ⅇ2⁢π⁢x-1)⁢ⅆ⁡x,