   SEARCH HOME Math Central Quandaries & Queries  Question from Arul: what is the easiest way to find the number which is both a square and a cube? the numbers i know are 64 and 729 which is both a sqr and a cube. i took long time to solve this.. is there any easier way? Hi Arul.

For a given positive number N to be a perfect square, it must satisfy S2 = N, where S is another positive number. Similarly, C3 = N for a perfect cube.

N = S2 = C3

Thus, if N is a 6th power, then this would certainly work. Say N = A6. Then S = A3 and C = A2.

So pick 6th powers. That's probably the easiest way to find solutions. Your two solutions are 64 = 26 and 729 = 36.

We can see that this is sufficient to determine it is a solution, but are all solutions 6th powers? To answer this use the fact that every number is the unique product of a set of prime numbers (this is technically called the "Fundamental Theorem of Arithmetic").

Consider that N is a number that satisfies N = S2 = C3. Then every prime factor that appears in the factorization of S appears twice as often in N, and there can be no other primes involved. So all N's prime factors appear an even number of times. Similarly, N's prime factors must appear an even "triple" of times to satisfy N = C3. Thus each factor in N appears an even number of times and the number of times is a multiple of 3. So every factor appears 6, 12, 18, ...
times. Thus, N is a 6th power. So we've shown that all solutions to your problem are 6th powers.

Hope this helps,
Stephen La Rocque.     Math Central is supported by the University of Regina and The Pacific Institute for the Mathematical Sciences.