Res_(z=c) {f(z)}=(1/(2πi)) ∮_( C) f(z)dz
Res_(z=1) {((z^(21) +z^2 +z+1)/((z−1)^3 ))}=(1/(2πi)) ∮_( C) ((z^(21) +z^2 +z+1)/((z−1)^3 ))dz
(1/(2πi)) ∮_( C) (((z^(21) +z^2 +z+1)/((z−1)^2 ))/(z−1))dz=lim_(z→1) ((z^(21) +z^2 +z+1)/((z−1)^2 ))
L′hosiptal :)
lim_(z→1) ((21z^(20) +2z+1)/(2(z−1))) and... Twice!!
lim_(z→1) ((420z^(19) +2)/2)=211
∴Res_(z=1) {f(z)}=211
★Caution★
f(α)′′=′′(1/(2πi)) ∮_( C) ((f(z))/(z−α)) dz
Why did I use big quotes for this
equation??
because the conditions for establshing
this equation are that path C
must be a simple closed curve
and there must be no singularity
in path C