We give a general method to construct a complete set of linearly independent
Casimir operators of a Lie algebra with rank N. For a Casimir operator of
degree p, this will be provided by an explicit calculation of its symmetric
coefficients $ g^{A_1,A_2,.. A_p}$. It is seen that these coefficients can be
descibed by some rational polinomials of rank N. These polinomials are also
multilinear in Cartan sub-algebra indices taking values from the set $I_0 =
{1,2,.. N}$. The crucial point here is that for each degree one needs, in
general, more than one polinomials. This in fact is related with an observation
that the whole set of symmetric coefficients $ g^{A_1,A_2,.. A_p} $ is
decomposed into sum subsets which are in one to one correspondence with these
polinomials. We call these subsets clusters and introduce some indicators with
which we specify different clusters. These indicators determine all the
clusters whatever the numerical values of coefficients $g^{A_1,A_2,.. A_p}$
are. For any degree p, the number of clusters is independent of rank N. This
hence allows us to generalize our results to any value of rank N.
To specify the general framework explicit constructions of 4th and 5th order
Casimir operators of $A_N$ Lie algebras are studied and all the polinomials
which specify the numerical value of their coefficients are given explicitly.