  Catalan conjecture/Mihailescu theorem is a theorem in number theory that was
conjectured by Mathematician Eugene Charles Catalan in 1844 and was proved
completely by Preda Mihailescu in 2005. Some form of problem dates back atleast
to Gersonides who seems to have proved a special case of the conjecture in
1343. The note stating the problem was not given the due imprtance at the
begining and appeared among errata to papers which had appeared in the earlier
volume of Crelle journal, however the problem got its due considration after
work of Cassles and Ko Chao in 1960s. The Catalan problem asks that the
equation $x^m-y^n=1$ has no solution for x,y,m,n in +ve integers other than the
trivial solution $ 3^2-2^3=1 $. An important and first ingredient for the proof
is Cassles criteria which says that whenever we have a solution of $x^p-y^q=1$
with p,q primes then $q|x$ and $p|y$ . Here we look a generalization of the
problem, namely we will consider the equation $x^p-y^q=1$ where x,y takes value
in ring of integers ${O}_K$ of a number field K and p,q are rational primes. In
this article we supply a possible formulation of Cassles criterion and a proof
for that in some particular cases of number fields. After this work one can
expect to follow Mihailescu and Characterize solutions of Catalan over number
fields.
