I know you're asking about a recursive call, but I don't think you need a recursive call here because the u calculated one time through the function is correct for the input n. It sounds like you mean you want to call this function multiple times for different n's stored in an array. To do that, you just have to return a value from the function and move the calling of the function out of the function definition (because, again, no recusion is necessary):
function u = mobius (n) % return u
if ~nargin % only prompt the user for n if it wasn't already given
n=input('Enter n');
end
% j=0;
p = factor(n);
N = hist([0 p],max(p)+1);
r = sum(N(2:end) > 1);
disp('The mobius value of'), disp(n), disp('is');
if (n == 1)
u = 1;
elseif (r > 0)
u = 0;
else
k = sum(N(2:end) > 0);
u = (-1)^k;
end
% a=[ 4 5 6 8 20];
% x=mobius(a(i))+mobius(a(j));
% disp(x);
end
So then, somewhere else (e.g., from the command line or in a different function or in a script), you call the function:
a=[ 4 5 6 8 20];
for i = 1:numel(a)
x=mobius(a(i));
disp(x);
end
