Suppose the positive integers and form an arithmetic progression such that a perfect cube and is a perfect square.
What is the number of digits in the smallest possible value of
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As a , b , c , d , e form an arithmetic progression we know that b + d = a + e = 2 c , and so
a + b + c + d + e = 5 c = m 3 for some positive integer m .
Again, since b + d = 2 c we have that b + c + d = 3 c = n 2 for some positive integer n .
So 3 c 5 c = n 2 m 3 ⟹ 5 n 2 = 3 m 3 .
To find the minimum value of c we need to keep m , n as small as possible. To this end, we need only have m , n have prime factors 3 and 5 . What remains to find is the multiplicities of these factors.
Letting n = 3 p 5 q and m = 3 r 5 s for non-negative integers p , q , r , s we then have that
5 ∗ ( 3 p 5 q ) 2 = 3 ( 3 r 5 s ) 3 ⟹ 3 2 p 5 2 q + 1 = 3 3 r + 1 5 3 s .
Equating powers of like exponents, we then require that 2 p = 3 r + 1 and 2 q + 1 = 3 s . The least allowed values satisfying these equations are p = 2 , r = 1 and q = 1 , s = 1 , giving us m = 3 ∗ 5 = 1 5 and n = 3 2 ∗ 5 = 4 5 .
Then as 5 c = m 3 we find that the minimum value for c is 5 1 5 3 = 3 ∗ 2 2 5 = 6 7 5 .
(To confirm, with 3 c = n 2 we find that c = 3 n 2 = 3 4 5 2 = 1 5 ∗ 4 5 = 6 7 5 . )