Overview What does “rare” really mean? In calculus, a number is called rare if it:is a non-palindromic integer and,has the property that n+r and n−r are both square, where r is the reversal of n (65 is the reversal of 56).so much term, right? But don’t worry, it’s straight forward definition once the terms are clear. Simply put, a palindromic number is a palindrome (a sequence of objects that stays the same when reversed), e.g., 77, 99, and 101. Thus, the sequence of rare numbers begins with ...