  We study Birkhoff sums S(k,a) = g(a)+g(2a)+...+g(ka) at the golden mean
rotation number a with periodic continued fraction approximations p(n)/q(n),
where g(x) = log(2-2 cos(2 pi x). The summation of such quantities with
logarithmic singularity is motivated by critical KAM phenomena. We relate the
boundedness of log- averaged Birkhoff sums S(k,a)/log(k) and the convergence of
S(q(n),a) with the existence of an experimentally established limit function
f(x) = lim S([x q(n)])(p(n+1)/q(n+1))-S([x q(n)])(p(n)/q(n)) for n to infinity
on the interval [0,1]. The function f satisfies a functional equation f(ax) +
(1-a) f(x)= b(x) with a monotone function b. The limit lim S(q(n),a) for n
going to infinity can be expressed in terms of the function f.
