You need to know the value of udisk for one k, like k=0 to define an end of the recursion. Assuming that udisk(0) = 1, the definition is straightforward
function u = udisk(k, tau, M)
if k == 1 % replace these two lines with the
u = 1 % actual definition of udisk
else
u = udisk(k-1, tau, M) - tau*M*sign(phi(k));
end
