have been written on exactly cards. On how many cards has the number 1024 been written on?
The natural numbers have been written on infinitely many cards so that all the divisors of the number
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We see that 1 is written once, then 2 is written once, 3 is written two times and 4 is written 2 times etc.It looks as if the number n is written on φ ( n ) cards.And indeed let it be true for all numbers less than n.Then the divisors of n would be written on
d ∣ n , d < n ∑ φ ( d ) = d ∣ n ∑ φ ( d ) − φ ( n ) = n − φ ( n ) cards.We used the well-known identity
d ∣ n ∑ φ ( d ) = n .
Then for n would be left
n − ( n − φ ( n ) ) = φ ( n ) cards.
What we are looking for is φ ( 2 1 0 ) = 2 9 = 5 1 2