IMO Shortlist 2005 problem N5
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Avg: 8,0 Denote by
the number of divisors of the positive integer
. A positive integer
is called highly divisible if
for all positive integers
.
Two highly divisible integers
and
with
are called consecutive if there exists no highly divisible integer
satisfying
.
(a) Show that there are only finitely many pairs of consecutive highly divisible
integers of the form
with
.
(b) Show that for every prime number
there exist infinitely many positive highly divisible integers
such that
is also highly divisible.
the number of divisors of the positive integer
. A positive integer
is called highly divisible if
for all positive integers
. Two highly divisible integers
and
with
are called consecutive if there exists no highly divisible integer
satisfying
. (a) Show that there are only finitely many pairs of consecutive highly divisible
integers of the form
with
. (b) Show that for every prime number
there exist infinitely many positive highly divisible integers
such that
is also highly divisible. Izvor: Međunarodna matematička olimpijada, shortlist 2005
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