Problem 3

n = 1 μ ( n ! ) = ? \large \displaystyle \sum_{n=1}^{\infty} \mu(n!) = \, ?

Notations :


The answer is 1.

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1 solution

Ameya Salankar
Apr 15, 2014

Notice that 2 2 = 4 n ! 2^2 = 4 | n! for n 4 n \geq 4 . Hence μ ( n ! ) = 0 \mu (n!) = 0 if n > 4 n > 4 . It follows that,

n = 1 μ ( n ! ) = μ ( 1 ! ) + μ ( 2 ! ) + μ ( 3 ! ) = 1 \displaystyle \sum_{n=1}^{\infty} \mu(n!) = \mu(1!) + \mu(2!) + \mu(3!) = 1 .

I liked this problem. Though, a reminder that μ represents the Möbius function would be nice!

Kevin Bourrillion - 7 years, 1 month ago

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@Kevin Bourrillion , OK!

Ameya Salankar - 7 years, 1 month ago

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