求帮忙

x0=[10,0.5,1]
x=lsqnonlin('ct',x0)
tt=0:1:16
yy=x(1).*(exp(-x(2).*tt-exp(-x(3).*tt)))
plot(tt,yy,'g')
hold on
t=[0.25 0.5 0.75 1 1.5 2 2.5 3 3.5 4 4.5 5 6 7 8 9 10 11 12 13 14 15 16];
c=[30 68 75 82 82 77 68 68 58 51 50 41 38 35 28 25 18 15 12 10 7 7 4];
plot(t,c,'*')
function y=ct(x)
t=[0.25 0.5 0.75 1 1.5 2 2.5 3 3.5 4 4.5 5 6 7 8 9 10 11 12 13 14 15 16];
c=[30 68 75 82 82 77 68 68 58 51 50 41 38 35 28 25 18 15 12 10 7 7 4];
y=c-x(1)*(exp(-x(2)*t)-exp(-x(3)*t))
x0 =
10.0000 0.5000 1.0000
??? Error using ==> lsqncommon at 101
LSQNONLIN cannot continue because user supplied objective function failed with the following
error:
Error using ==> feval
Undefined function or method 'ct' for input arguments of type 'double'.
Error in ==> lsqnonlin at 181
[x,Resnorm,FVAL,EXITFLAG,OUTPUT,LAMBDA,JACOB] = ...

改过之后，可是还不行