a(n) = m if m has already occurred at least once and n=k+i*(m+1) where k is the index of the first occurrence of n and i=1,...,max(n-1,1), otherwise a(n) = least positive integer that has not yet occurred.

a(16) does not exist: it would have to be 4 (since a(6) = a(11) = 4 and so a(6 + 2*(4 + 1)) = a(16) = 4) and also 6 (since a(9) = 6 and so a(9 + 1*(6 + 1)) = a(16) = 6) simultaneously. - Herman Jamke (hermanjamke(AT)fastmail.fm), May 01 2008