Monday 16 November 2020

Harmonic differences

Do there exists two positive integers hh and ll such that i=lh1i\sum_{i = l}^h \frac{1}{i} is an integer other than the trivial case?

Every natural number can be written as 2αx2^\alpha x. More specifically for each number nn, α\alpha is the n-th element of this sequence, and xx is the n-th term of this sequence.
Let us first have a look at α\alpha. If you take a consecutive subset, from index ll to hh, the maximum always appears ones.

Proof:

Assume the maximum yy appears at least twice. As the index of the number has to be of the form 2αx2^\alpha x, the two appearances can be assumed to be 2ya2^y a and 2yb2^y b with aa and bb odd and a<ba < b. As 2ya<2y(a+1)<2yb2^y a < 2^y (a+1)< 2^y b, and both ends are in the consecutive subset, $ 2^y (a+1)$ has to be in the subset. But as aa is odd, a+1a+1 is even, and such a+1a+1 can be written as 2kz2^k z with k nonzero and zz odd. Hence, 2y(a+1)=2y+kz2^y (a+1) = 2^{y+k} z, so there exists a value y+ky+k larger than yy for the exponent, so yy was not the maximum, which leads to a contradiction.

Now, take the sum i=lh1i\sum_{i = l}^h \frac{1}{i}. By the previous lemma, there is one single element that contains the largest power of two, if written like 2αx2^\alpha x. Remove that element from the sum. The sum of all other elements can be written as plcm(i)\frac{p}{\text{lcm} (i)} with aa an integer and lcm(i)\text{lcm} (i) the least common multiple of all denominators. As all of the denominators have a lower exponent for two than α\alpha, lcm(i)\text{lcm} (i) is not a multiple of α\alpha. Let us write this is 2βy2^\beta y with β<α\beta < \alpha

Our total sum is now 12αx+p2βy=2βy+2αxp2αx2βy\frac{1}{2^\alpha x} + \frac{p}{2^\beta y} = \frac{2^\beta y + 2^\alpha x p}{2^\alpha x 2^\beta y}
As the denominator is a multiple of 2α2^\alpha and the numerator is not, this can never be an integer. QED