Function handle of a sum

How can I define a function that involves a summation?

for example, a function f = @(x) e^(-n*x), where the function is a sum of terms over some range of n?

Thanks

[Bernoulli Lizard]

Actually it's slightly more complicated than that. I need the function to be of another function of n, so something of the form

f = @(x) exp( -g(n) * x ), summed over some range of n.

Answer 1

[Matt J]

Assuming g(n) has vectored input/output, just do

 f=@(x) sum( exp( -g(1:n) .* x ));  %sum over 1...n

with obvious modifications for different ranges of n.

[Bryan Engelhardt]

The function doesn't work for me when integrating. In other words,

    g = [1,2]
    f = @(x) sum( exp( -g(1:2) .* x));
    integral(f,0,1);

gives me a matrix dimension error. How do I fix this?

[Matt J]

    g = [1,2]
    f = @(x) sum( exp( -g(1:2) .* x));
    F=  @(X) arrayfun(f,X);
    integral(F,0,1);

or

    integral(f,0,1,'ArrayValued',1);