function y = c_ext(x,epsilon,alpha,k,np)
rp=4*x*np;

Qa=1+2*exp(-rp)./rp+2*(exp(-rp)-1)./rp.^2;
y = Qa.*pi.*x.^2./k.^2.*epsilon^4.*alpha.^5./(epsilon.^2-1)./x.^5.*sqrt((epsilon.^2-1)./(alpha^2*epsilon.^2-x.^2));
