Numerator of p-1th Harmonic Number is Divisible by Prime p/Proof 2
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Theorem
Let $p$ be an odd prime.
Consider the harmonic number $H_{p - 1}$ expressed in canonical form.
The numerator of $H_{p - 1}$ is divisible by $p$.
Proof
Note that for any integer $x$:
\(\ds x^p - x\) | \(\equiv\) | \(\ds 0\) | \(\ds \pmod p\) | Corollary $1$ to Fermat's Little Theorem | ||||||||||
\(\ds \) | \(\equiv\) | \(\ds x^{\overline p}\) | \(\ds \pmod p\) | Divisibility of Product of Consecutive Integers | ||||||||||
\(\ds \) | \(\equiv\) | \(\ds \sum_k {p \brack k} x^k\) | \(\ds \pmod p\) | Sum over k of Unsigned Stirling Numbers of First Kind by x^k |
By comparing coefficients:
- $\ds {p \brack p} \equiv 1 \pmod p$
- $\ds {p \brack 1} \equiv -1 \pmod p$
- $\ds {p \brack k} \equiv 0 \pmod p$ for $k \ne 1, p$
or in a more compact form:
- $\ds {p \brack k} \equiv \delta_{k p} - \delta _{k 1} \pmod p$
where:
- $\ds {p \brack k}$ denotes an unsigned Stirling number of the first kind
- $\delta$ is the Kronecker delta.
From Harmonic Number as Unsigned Stirling Number of First Kind over Factorial:
- $\ds H_{p - 1} = \frac {p \brack 2} {\paren {p - 1}!}$
From the above we have:
- $\ds p \divides {p \brack 2}$
By Prime iff Coprime to all Smaller Positive Integers we also have:
- $p \nmid \paren {p - 1}!$
Hence the numerator of $H_{p - 1}$, when expressed in canonical form, is divisible by $p$.
$\blacksquare$
Sources
- 1997: Donald E. Knuth: The Art of Computer Programming: Volume 1: Fundamental Algorithms (3rd ed.) ... (previous) ... (next): $\S 1.2.7$: Harmonic Numbers: Exercise $17$