function y = x2cdf(x,k)
% Evaluates the cumulative distribution function of the chi-square
% distribution at x with k df.

alpha = k/2;

beta = 2;

a = zeros(alpha,1);

for ii = 0:alpha-1
    if ii == 0
        a(ii+1) = (x/beta)^ii;
    else
        a(ii+1) = (x/beta)^ii / prod(1:ii);
    end
end

y = 1 - exp(-x/beta) * sum(a);