Tara - try replacing the switch with an if and use a tolerance check (note that using a switch statement for doubles or floats will not work as expected because of how floating point numbers are represented - see Floating-Point Numbers for more details)
pa=2*0.11*(1-(F(3)-Fa0)/Fa0);
pc=2*0.11*(F(3)-Fa0)/Fa0;
% the amount of pc should be checked .if pc is less than eps it should be
% replaced with "1" in ''sqrt(pa/pc)'' term.
if abs(pc) < eps
ra=-k.*sqrt(pa).*(pa-(pc/kp.*pa).^2);
else
ra=-k.*sqrt(pa/pc).*(pa-(pc/kp.*pa).^2);
end
Note tha I changed the above code to use eps instead of 0.1 which may or may not be appropriate for your code.
