Take the 2-minute tour ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

show that $$(p-1)!\left(1+\dfrac{1}{2}+\dfrac{1}{3}+\cdots+\dfrac{1}{p-1}\right)\equiv 0(\mod p^2)$$

maybe use this $$\dfrac{1}{k}+\dfrac{1}{p-k}=\dfrac{p}{k(p-k)}$$ and then I can't,can you help me to prove it? Thank you

share|improve this question
1  
Adapting Wilson's theorem mod $p^2$? –  GIANCANE Nov 6 '13 at 12:09
    
I think the general expression is not an integer; at least for p=3,5. –  user99680 Nov 6 '13 at 12:15
1  
It is, since every denominator appears as a factor of $(p-1)!$. –  Christoph Nov 6 '13 at 12:16
2  
More is true: en.wikipedia.org/wiki/Wolstenholme%27s_theorem. –  lhf Nov 6 '13 at 12:17
    
Ah, yes, I was adding incorrectly. –  user99680 Nov 6 '13 at 12:18

2 Answers 2

up vote 4 down vote accepted

The solution below is adapted from Notes on Wolstenholme’s Theorem by Timothy H. Choi.

Let $$ S=(p-1)!\sum_{k=1}^{p-1} \frac1k $$ Using your insight $$ \dfrac{1}{k}+\dfrac{1}{p-k}=\dfrac{p}{k(p-k)} $$ we have $$ 2S=(p-1)!\sum_{k=1}^{p-1} \left(\dfrac{1}{k}+\dfrac{1}{p-k}\right) = p\sum_{k=1}^{p-1} \frac{(p-1)!}{k(p-k)} = pS' $$ Note that $S'$ is an integer. Now $$ \frac{(p-1)!}{k(p-k)} \equiv (k^2)^{-1} \bmod p $$ where the inverse is taken ${}\bmod p$. This is a consequence of Wilson’s Theorem. Hence $$ S'\equiv \sum_{k=1}^{p-1} (k^2)^{-1} \equiv \sum_{k=1}^{p-1} k^2 = \frac{(p-1)p(2(p-1)+1)}{6} \equiv 0 \bmod p $$ This means that $2S\equiv 0 \bmod p^2$ and so $S\equiv 0 \bmod p^2$. (We need $p>3$ twice here.)

share|improve this answer

As others have noted, the congruence is not true for $p=3$, since $$ 2!\left(1+\frac 1 2\right)=2+1=3,$$ which is not divisible by $9$. We can still use what you suggested to prove the congruence holds $\operatorname{mod} p$. Let $p$ be an odd prime, then \begin{align*} (p-1)!\sum_{k=1}^{p-1} \frac 1 k &= (p-1)! \sum_{k=1}^{(p-1)/2} \left(\frac 1 k + \frac 1 {p-k}\right) \\&= (p-1)!\sum_{k=1}^{(p-1)/2} \frac{p}{k(p-k)} = p\sum_{k=1}^{(p-1)/2} \frac{(p-1)!}{k(p-k)}. \end{align*} Since $(p-1)!$ always contains $k$ and $(p-k)$ as a factor, the fractions in the sum are integers and the result is a multiple of $p$.

See lhf's answer for why it is even a multiple of $p^2$ as long as $p>3$.

share|improve this answer
    
The congruence will hold for any $p>3$ modulo $p^2$. That's Wolstenholme's theorem as lhf pointed out. –  EuYu Nov 6 '13 at 12:39
1  
I adapted my post and referencered lhf's answer, thanks for your comment! –  Christoph Nov 6 '13 at 12:54

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.