Your function theiss calls itself with one input; that's the reason for the error.
function [ux] = theiss(r,S,T,t)
% theiss calls theiss here, with one input, which has value (r^2 *S)/(4 * T * t):
ux = theiss((r^2 *S)/(4 * T * t));
% so in that call to theiss, arguments S, T, and t are not given and you get
% the error when it tries to do (r^2 *S)/(4 * T * t) for the next call to
% theiss ...
end
Perhaps the function should be defined as:
function [ux] = theiss(r,S,T,t)
% just calculate ux and return it
ux = (r^2 *S)/(4 * T * t);
end
