Denote by R(N) the integer obtained by reversing the digits of a positive integer N.
Determine the largest integer that is certain to
divide N4 - (R(N))4, with N > R(N), regardless of the choice of N.
x^4-y^4= (x^2-y^2)* (x^2+y^2) and
(x^2-y^2)= (x-y)* (x+y)*
x= 1000a+ 100b+10c+d
x-y is divisible by 9
x+y is divisible by 11
ergo the product is divisible by 99 for any x and y if one is reverse of the other
x^2+y^2 contributes nothing