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Extend computation to 2^50, and give heuristic for expected count of "!" records
Noam D. Elkies
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[Edited mostly to extend the computation from $1.5 \cdot 10^{13}$ to a bit over $2^{50} > 10^{15}$ and give the heuristics for expected number of records for $\| r(n) \|$ vs. $\log n \cdot \| r(n) \|$]

Just ran across this. I see that Kevin's answer completely settles the original question, but meanwhile Will Jagy raised the question of finding new record lows for $$ \log n \cdot \left\| \frac{n}{\log n} \right\| $$ and proving their infinitude. I next outline a proof that there are infinitely many such record lows, and then report on a computation of all such $n$ up to $1.5 \cdot 10^{13}$.

For the infinitude: Since $r(n) := n / \log n$ can never be an exact integer, it is enough to prove that for each $\epsilon > 0$ there exist infinitely many solutions of $\| r(n) \| < \epsilon/\log n$. In fact it's not hard to show that $\| r(n) \|$ can get as small as some negative power of $n$, because $r(n)$ is almost linear (its second derivative is $o(n^{-1})$ as $n \rightarrow \infty$) and we can choose $n_0$ to make $r'(n_0)$ as far as possible from any rational number. If I did this right, we can find intervals $|n - n_0| \leq h$ in which $\min_n \| r(n) \| \ll h^{-1}$ where $h^{-1} = |r''(n_0)|^{1/3} \sim (n_0 \log^2 n_0)^{-1/3}$. For instance, we may choose $n_0$ so that $r'(n_0) = 1 / (k + \sqrt 2)$ for $k = 1, 2, 3, \ldots$ [that is, so that $\log n_0$ solves the quadratic equation $\lambda^2 = (k+\sqrt2) (\lambda-1)$]. On such an interval, $r(n)$ is approximated by $r(n_0) + r'(n_0)(n-n_0)$ to within $O(r''(n_0) (n-n_0)^2) = O(h^2/h^3) = O(h^{-1})$, and (since $h$ grows much faster than $k$) the arithmetic sequence with common difference $r'(n_0)$ is close enough to being equidistributed that it comes within $O(1/h)$ of an integer. [We probably expect that $\| r(n) \|$ is random enough that it gets as small as $c/n$ or even $o(1/n)$, but proving such a result must be well out of reach.]

For the numerical search, the problem is quite similar to MO.19170 on nearly-integral values of $\log_{10} n!$ (since $n/ \log n$, like $\log_{10} n!$, is nearly linear in $n$). Again it takes time only $\tilde O(N^{2/3})$ to find all examples with $n < N$ using a linear-approximation technique such as described at the bottom of page 15 of Lefèvre's slides. This is actually the same idea as in the previous paragraph: partition $[1,N]$ into intervals $|n-n_0| \leq h \sim (n_0 \log^2 n_0)^{1/3}$; in general $r'(n_0)$ might be so close to a rational number that equidistribution fails, but we can still use continued fractions to find all $n$ in that interval for which $\|r(n)\| \ll h^{-1}$.

I ran this with $N = 2^{50} > 10^{15}$ on ten alhambra heads. Most finished in under two days; two took an extra day or two, probably spending most of them on $n_0$ for which $r'(n_0)$ was nearly rational (in this case one can do much better than trying every $n \in [n_0-h,n_0+h]$ for which $\| r(n_0) + r'(n_0)(n-n_0) \|$ is small, but I didn't take the extra time to implement that refinement). The computation found fourteen new records beyond the 12 initial terms 2, 17, 163, 715533, 1432276, 6517719, 11523158, 11985596, 24102781, 254977309, 451207448, 1219588338 of OEIS sequence A178806, namely

2048539023, 10066616717, 42116139191, 47657002570, 73831354169, 122478947521, 143949453227, 3152420311977, 5624690531099, 14964977749017, 25999244327633, 92799025313425, 164330745650026, and 604329910739082.

There is also a new example, namely $n = 3040705645816$, of a number that is not in this sequence but does belong in the closely related OEIS sequence A178805, which consists of $n$ that achieve record low values of $\| r(n) \|$ instead of $\log n \cdot \| r(n) \|$. In general a $\log n \cdot \| r(n) \|$ record is automatically also an $\| r(n) \|$ record, but the converse can fail on occasion. If we imagine that the $\| r(n) \|$ are independent random numbers uniformly distributed on $(0,1/2)$ then the probability that $\| r(n) \|$ is a new record is $1/n$, so we expect $\log N + O(1)$ record values with $n \leq N$. The same question for $\log n \cdot \| r(n) \|$ is trickier, but if I did this right the probability that $\| r(n) \|$ is a new record but $\log n \cdot \| r(n) \|$ is not one is approximately $1 / n \log n$, so we expect only $\log\log N + O(1)$ examples such as $n = 3040705645816$ up to $N$, and might never see another one even though there should be infinitely many more.

Here is a table of the values of $n < 2^{50}$ for which $\| r(n) \|$ attains a new record low, together with the signed fractional part of $r(n)$, and $\log n$ times that fractional part: $$ \begin{array}{rrrc} 2 & -0.1146099 & -0.0794415 & \\ 5 & 0.1066747 & 0.1716863 & ! \\ 9 & 0.0960765 & 0.2111017 & ! \\ 13 & 0.0683262 & 0.1752532 & ! \\ 17 & 0.0002541 & 0.0007199 & \\ 163 & -1.26 \cdot 10^{-6} & -6.43 \cdot 10^{-6} & \\ 53453 & 1.22 \cdot 10^{-6} & 1.33 \cdot 10^{-5} & ! \\ 110673 & 6.68 \cdot 10^{-7} & 7.76 \cdot 10^{-6} & ! \\ 715533 & 3.84 \cdot 10^{-7} & 5.17 \cdot 10^{-6} & \\ 1432276 & 2.33 \cdot 10^{-7} & 3.30 \cdot 10^{-6} & \\ 6517719 & -2.00 \cdot 10^{-7} & -3.14 \cdot 10^{-6} & \\ 11523158 & -9.95 \cdot 10^{-8} & -1.62 \cdot 10^{-6} & \\ 11985596 & -7.26 \cdot 10^{-8} & -1.18 \cdot 10^{-6} & \\ 24102781 & 4.43 \cdot 10^{-9} & 7.53 \cdot 10^{-8} & \\ 254977309 & 9.12 \cdot 10^{-10} & 1.76 \cdot 10^{-8} & \\ 451207448 & 3.68 \cdot 10^{-10} & 7.33 \cdot 10^{-9} & \\ 1219588338 & -2.57 \cdot 10^{-10} & -5.38 \cdot 10^{-9} & \\ 2048539023 & -5.89 \cdot 10^{-11} & -1.26 \cdot 10^{-9} & \\ 10066616717 & 4.85 \cdot 10^{-11} & 1.12 \cdot 10^{-9} & \\ 42116139191 & -4.47 \cdot 10^{-11} & -1.09 \cdot 10^{-9} & \\ 47657002570 & -2.43 \cdot 10^{-11} & -5.97 \cdot 10^{-10} & \\ 73831354169 & 1.35 \cdot 10^{-11} & 3.38 \cdot 10^{-10} & \\ 122478947521 & 7.53 \cdot 10^{-13} & 1.92 \cdot 10^{-11} & \\ 143949453227 & -5.50 \cdot 10^{-13} & -1.41 \cdot 10^{-11} & \\ 3040705645816 & 5.18 \cdot 10^{-13} & 1.49 \cdot 10^{-11} & ! \\ 3152420311977 & -3.36 \cdot 10^{-13} & -9.67 \cdot 10^{-12} & \\ 5624690531099 & 1.28 \cdot 10^{-13} & 3.76 \cdot 10^{-12} & \\ 14964977749017 & -7.15 \cdot 10^{-14} & -2.17 \cdot 10^{-12} & \\ 25999244327633 & -2.02 \cdot 10^{-14} & -6.25 \cdot 10^{-13} & \\ 92799025313425 & 6.01 \cdot 10^{-15} & 1.93 \cdot 10^{-13} & \\ 164330745650026 & -1.00 \cdot 10^{-15} & -3.28 \cdot 10^{-14} & \\ 604329910739082 & -4.59 \cdot 10^{-16} & -2.27 \cdot 10^{-14} & \end{array} $$ the "!"'s mark the $\| r(n) \|$ records that aren't $\log n \cdot \| r(n) \|$ records.

Noam D. Elkies
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