# Solve $p(n)^2=\sigma(n)+423$ for $1<n<2012$

How can I solve the equation $p(n)^2=\sigma(n)+423$ for $1<n<2012$, where $p(n)$ is the product of distinct prime divisors of $n$ and $\sigma(n)$ is the sum of divisors of $n$.

I did this: $n=p_1^{a_1}\cdot p_2^{a_2}\cdots p_k^{a_k}$, then $p(n)=p_1\cdots p_k$ and $\sigma(n)=\frac{p_1^{a_1+1}-1}{p_1-1}\cdots \frac{p_k^{a_k+1}-1}{p_k-1}$. Then we have $$(p_1\cdots p_k)^2=\frac{p_1^{a_1+1}-1}{p_1-1}\cdots\frac{p_k^{a_k+1}-1}{p_k-1}+423.$$

But now I'm stuck :(. Help me please.

• $\sigma(.)$ is the standard notation for the sum of divisors.
– Xam
Apr 3 '17 at 20:19
• What is the source of this problem? Apr 17 '17 at 11:11

Short answer: A simple program can show that $n=1377$ is the only number less than $100000$ satisfying $p(n)^2=\sigma(n)+423$

Lemma 1: $n$ is odd number.

Suppose $n$ is even. Then we can observe that $p(n)^2-423=\sigma(n)$ is odd. Therefore, each $(p_i^{a_i+1}-1)/(p_i-1)=1+p_i+\cdots+p_i^{a_i}$ is odd. There are two possibilities to make this number odd: i) $p_i=2$, ii) $a_i$ is even. That is, $n=2^am^2$ for some positive integer $a$ and odd number $m$.

Now, suppose that there is an odd prime $p_j$ such that $a_j \ge 4$. Then, $$n+423<\sigma(n)+423=p(n)^2=\prod_{i=1}^k p_i^2\le\frac{2}{p_j^2}\prod_{i=1}^k p_i^{a_i}\le \frac{2n}{9}$$this is contradiction. Also, if exponent of $2$ is greater than $1$, then$$n+423<\sigma(n)+423=p(n)^2=\prod_{i=1}^k p_i^2\le\prod_{i=1}^k p_i^{a_i}=n$$and this is also contradiction. Therefore, $a_i=1$ if $p_i=2$, $a_i=2$ otherwise. It follows that $p(n)^2=2n$.

Also note that if $3|n$, then $2n=p(n)^2>\sigma(n)\ge n+n/2+n/3+n/6=2n$, which is a contradiction.

i) $k=1$: $n=2$, impossible.

ii) $k=2$: $n=2p^2$ for some odd prime $p$. But the equation $3(p^2+p+1)+423=4p^2$ has no integer root.

iii) $k \ge 3$: $n\ge2 \times 5^2 \times 7^2>2016$, so no solution.

Therefore, $n$ must be odd number.

Lemma 2: $k > 1$. That is, $n$ is not $1$, prime or prime power.

It is easy to see that $n$ is not $1$. Let $n=p^a$ for some prime $p$. Then,$$p^2=p(n)=\sigma(n)+423=1+\cdots+p^a+423$$and if $a\ge 2$, it is obvious that$$p^2<1+\cdots+p^a+423$$. Therefore $a=1$. But, then $p^2=1+p+423$ and one can check that the quadratic equation has no integer root.

Lemma 3: $k<3$.

Firstly, if $k \ge 5$, then $n \ge 3*5*7*11*13 \ge 2016$. Therefore $k \le 4$.

Recall that$$\frac{\sigma(n)}{n}=\prod_{i=1}^k \left(1+\frac{1}{p_i}+\cdots+\frac{1}{p_i^{a_i}}\right)<\prod_{i=1}^k \left(1+\frac{1}{p_i-1}\right)=\prod_{i=1}^k \left(\frac{p_i}{p_i-1}\right)$$ if $k\ge 3$, then by lemma 1, $p(n)^2\ge(3*5*7)^2>10000$ and $\sigma(n)+423<n*(3/2)*(5/4)*(7/6)*(11/10)+423<n*2.5+423<2012*2.5+423<10000$ therefore there is no $n$ satisfying the equation.

The solution to problem itself

By lemma 2 and 3, $k=2$. Let $n=p^aq^b$ where $p$ and $q$ are primes and $p<q$ without loss of generality. By lemma 1, both $p$ and $q$ are odd. By processing RHS, we get$$\sigma(n)+423<n\frac{p}{p-1}\frac{q}{q-1}+423<2012\frac{3}{2}\frac{5}{4}+423=4195.5$$and we get $pq=p(n)<\sqrt{4195.5}<65$. Therefore, there are limited number of possible $(p,q)$ pair: $(3,5), (3,7), (3,11), (3,13), (3,17), (3,19), (5,7), (5,11)$. Let's check all these cases one by one. Note that $\sigma(n)=p^2q^2-423$.

i) $(p,q)=(3,5)$: $\sigma(n)=-198$. No solution.

ii) $(p,q)=(3,7)$: $\sigma(n)=18$. $21\le n<\sigma(n)=18$. No solution.

iii) $(p,q)=(3,11)$: $\sigma(n)=666$. $666$ is multiple of $37$, therefore $37|(3^{a+1}-1)$ or $37|(11^{b+1}-1)$, and $3^a<666$, $11^b<666$. However, there exists no such $a$ or $b$ which satisfies these conditions.

iv) $(p,q)=(3,13)$: $\sigma(n)=1098$. $1098$ is multiple of $61$, therefore $61|(3^{a+1}-1)$ or $61|(13^{b+1}-1)$, and $3^a<1098$, $13^b<1098$. The only $a$ or $b$ satisfying the condition is $b=2$. But now we get $\sigma(3^a)=6$, which is impossible.

v) $(p,q)=(3,17)$: $\sigma(n)=2178$. $2178$ is multiple of $11^2$, but there is no $b$ such that $11|(17^{b+1}-1)$ and $17^b<2178$. On the other hand, the only $a$ satisfying $11|(3^{a+1}-1)$ and $3^a<2178$ is $a=4$. Now we get $\sigma(17^b)=18$, which gives $b=1$ and $n=1377$.

vi) $(p,q)=(3,19)$: $\sigma(n)=2826$. $2826$ is multiple of $157$, therefore $157|(3^{a+1}-1)$ or $157|(19^{b+1}-1)$, and $3^a<2826$, $19^b<2826$. However, there exists no such $a$ or $b$ which satisfies these conditions.

vii) $(p,q)=(5,7)$: $\sigma(n)=802$. $802$ is multiple of $401$, therefore $401|(5^{a+1}-1)$ or $401|(7^{b+1}-1)$, and $5^a<802$, $7^b<802$. However, there exists no such $a$ or $b$ which satisfies these conditions.

viii) $(p,q)=(5,11)$: $\sigma(n)=2602$. $2602$ is multiple of $1301$, therefore $1301|(5^{a+1}-1)$ or $1301|(11^{b+1}-1)$, and $5^a<2602$, $11^b<2602$. However, there exists no such $a$ or $b$ which satisfies these conditions.

Therefore, $n=1377$ is the only solution.

• Thank you but I'm not allowed to use programs to solve the problem :( Apr 3 '17 at 19:46