c=[-2:.1:2];

lp=(c+sqrt(c.^2+4))/2;
lm = (c-sqrt(c.^2+4))/2;
mp = (c+sqrt(c.^2-4))/2;
mm = (c-sqrt(c.^2-4))/2;

rp=(mp+lm)./(mp+lp);
rm = (lm+mm)./(lp+mm);
f=(mp.*log(-rp)-mm.*log(-rm))./(mp-mm);
alf = exp(f)./(1+exp(f));

figure(1)
plot(alf,c,[0,1],[0,0],'--','linewidth',2)
xlabel('\alpha','fontsize',20)
ylabel('c','fontsize',20)
