Error! Cannot compute the sum with respect to '0.75'. The summation index must be a symbolic variable

Expression 1-(8/pi^2)*symsum((1/(2*n+1)^2).*exp(-(D*(2*n+1)^2*pi^2.*x)/(2*h)^2),n,0,inf) is not a valid MATLAB expression, has non-scalar coefficients, or cannot be evaluated:

Error in fittype expression ==> 1-(8./pi.^2).*symsum((1./(2.*n+1).^2).*exp(-(D.*(2.*n+1).^2.*pi.^2.*x)./(2.*h).^2),n,0,inf)

??? Cannot compute the sum with respect to '0.75'. The summation index must be a symbolic variable.

When use the cftool toolbox,and the n is defined as syms n

[KSSV]

Show us the full code which you have tried.

[Yp Wu]

clear
clc
x1=[1439 1379 1319 1259 1199 1139 1079 1019 959 899 839	779	719	659	599	539	479	419	359	299	239	179	119 0];
x=fliplr(x1); 
y1=[1.168569611 1.161925842 1.132080012	1.101075755	1.059439993	1.009530311	0.961292398	0.945544204	0.907838728	0.86918407	0.848133369	0.828202061	0.806753448	0.785278638	0.720429083	0.681182912	0.643804954	0.621425941	0.581850938	0.535329812	0.468631579	0.35888064	0.253023249 0];
y=fliplr(y1);
mq=1.1;
h=0.2;
syms D n;
y2=vpa(1-(8/pi^2)*symsum((1/(2*n+1)^2)*exp(-(D*(2*n+1)^2*pi^2*sqrt(x))/(2*h)^2),n,0,inf),3); %need fitting use the cftool
cftool

[Torsten]

Tell cftool that you supply your function in a user-written routine named e.g. infinite_sum:

ft = fittype( 'infinite_sum( x, D, h )' )

and in this function, return the value of the infinite sum as a double, not as a symbolic expression.

[Yp Wu]

Thank you very much for your reply, I understand what you mean, and tried in this way, another error is the symsum formula has too many variables.

[Torsten]

If you work with a numerical fitting tool like cftool, you should also work with the numerical "sum" instead of the symbolic "symsum".