n → ∞ lim [ f ( H 1 0 n ) ] − 1 0 n 1
Let f be the Thomae function and H n be the nth harmonic number. Evaluate the limit above.
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You are posting really nice problem these days :D
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Thank you! I'll try to post problems with original research one of these days!
Have you yourself derived this result.
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See the comments here .
Notice that f ( H k ) 1 is the denominator of H k , since H k is always rational. To make things convenient, let's call 1 / f ( x ) = d ( x ) for denominator.
Now, what do we usually do with weird-looking limits? Take their logarithm, of course!
lo g 1 0 n → ∞ lim f ( H 1 0 n ) − 1 / 1 0 n = n → ∞ lim 1 0 n lo g 1 0 d ( H 1 0 n )
Now, we know that ⌈ lo g 1 0 N ⌉ is the number of decimal digits of N = 1 0 n for some positive integral n . Since d ( H 1 0 n ) → ∞ as n → ∞ , we can just ignore the ceiling function. If d # ( N ) is the number of decimal digits of N , then
n → ∞ lim 1 0 n lo g 1 0 d ( H 1 0 n ) = n → ∞ lim 1 0 n d # ( H 1 0 n ) = lo g 1 0 e
which is a consequence of this sequence 's properties.
So, we can finally conclude
n → ∞ lim f ( H 1 0 n ) − 1 / 1 0 n = 1 0 lo g 1 0 e = e = 2 . 7 1 8 …