A special number is a positive integer for which there exist positive integers and with
Prove that:
(a) there are infinitely many special numbers;
(b) 2014 is not a special number.
A special number is a positive integer for which there exist positive integers and with
Prove that:
(a) there are infinitely many special numbers;
(b) 2014 is not a special number.
(a) Every perfect cube of a positive integer is special because we can write
for some positive integers .
(b) Observe that . If 2014 is special, then we have,
for some positive integers . We may assume that is minimal with this property. Now, we will use the fact that if 19 divides , then it divides both and . Indeed, if 19 does not divide , then it does not divide too. The relation implies . The latter congruence is equivalent to . Now, according to Fermat's Little Theorem, we obtain , that is 19 divides 63, not possible.
It follows , for some positive integers and . Replacing in (1) we get
i.e. . It follows and , and replacing in (2) we get
Clearly, , contradicting the minimality of .