2005 AMC 10A Problems/Problem 24
Contents
Problem
For each positive integer , let
denote the greatest prime factor of
. For how many positive integers
is it true that both
and
?
Solution 1
If , then
, where
is a prime number.
If , then
is a square, but we know that n is
.
This means we just have to check for squares of primes, add and look whether the root is a prime number.
We can easily see that the difference between two consecutive square after
is greater than or equal to
,
Hence we have to consider only the prime numbers till
.
Squaring prime numbers below including
we get the following list.
But adding to a number ending with
will result in a number ending with
, but we know that a perfect square does not end in
, so we can eliminate those cases to get the new list.
Adding , we get
as the only possible solution.
Hence the answer is
.
edited by mobius247
Note: Solution 1
Since all primes greater than are odd, we know that the difference between the squares of any two consecutive primes greater than
is at least
, where p is the smaller of the consecutive primes. For
,
. This means that the difference between the squares of any two consecutive primes both greater than
is greater than
, so
and
can't both be the squares of primes if
and
. So, we only need to check
and
.
~apsid
Video Solution
~rudolf1279
Solution 2
If , then
, where
is a prime number.
If , then
, where
is a different prime number.
So:
Since :
.
Looking at pairs of divisors of , we have several possibilities to solve for
and
:
The only solution where both numbers are primes is
.
Therefore the number of positive integers that satisfy both statements is
Solution 3
For the statement to be true, we must have both and
be squares of primes. Support we have the number
, where
is a positive integer. Then the next perfect square,
, is
greater than
. The next perfect square after that will be
greater than
. In general, the prime
will be
greater than
. However, we must have that
.
can take on any value between
and
(if
is equal to
, we have
, where
would have to be negative for the difference to be
). However, we can eliminate all the cases where
is odd, because we would then have a number of the form
, which is odd because
can take only integral values. As such, we consider
,
, and
. If
, then
. Then our squares are
and
, both of which are squares of primes. If
, then
. However,
isn't prime, so we discard this case. Finally, if
, then
. Again,
isn't prime, so we discard this case as well. Thus, we only have
valid case.
~ cxsmi
Video Solution 2
~savannahsolver
See Also
2005 AMC 10A (Problems • Answer Key • Resources) | ||
Preceded by Problem 23 |
Followed by Problem 25 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
All AMC 10 Problems and Solutions |
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