How many solutions are there to the equation where and is a positive integer?
Notation:
is the
factorial
notation. For example,
.
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n ! = k n 3 = > ( n − 1 ) ! = k n 2 .
It is obvious that no prime will satisfy the above equation because ( n − 1 ) ! does not contain the prime n .
So, at least n will be a composite number.Let n = p 1 k 1 p 2 k 2 . . . . . p r k r ⟹ n 2 = p 1 2 k 1 p 2 2 k 2 . . . . . p r 2 k r .So, in the ( n − 1 ) ! all these factors should be present.Let us assume that m a x { p 1 , p 2 , p 3 , , , , , p r } = p r ..To satisfy the equation we must have
2 k r p r ≤ ( n − 1 )
With the help of these condition we can easily find solutions in the given range.