147% Perfect

The sum of all the divisors of x x , a positive integer, are summed, the result is 3 x 1 2 \frac{3x-1}{2}

Find the sum of the 7 7 smallest possible values of x x .


The answer is 1093.

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3 solutions

Ankit Kumar Jain
Apr 27, 2015

The numbers are powers of 3 3 .

P R O O F : \underline{PROOF:-}

Let N = a p × b q × c r × d s N = a^{p}\times{b^{q}}\times{c^{r}}\times{d^{s}}

Then sum of divisors are ( a p + 1 1 ) × ( b q + 1 1 ) × ( c r + 1 1 ) × ( d s + 1 1 ) ( a 1 ) × ( b 1 ) × ( c 1 ) × ( d 1 ) \dfrac{{(a^{p+1} - 1)}\times{(b^{q+1} - 1)}\times{(c^{r+1}-1)}\times{(d^{s+1}-1)}}{{(a-1)}\times{(b-1)}\times{(c-1)}\times{(d-1)}} .

So now let N be x = 3 n x = 3^{n} , then the sum of divisors = 3 n + 1 1 3 1 \dfrac{3^{n+1}-1}{3-1} .

Replacing x = 3 n x = 3^{n} ,

The sum of divisors = 3 x 1 2 = \dfrac{3x-1}{2} .

Parth Bhardwaj
Mar 10, 2015

I just started listing them and noticed that the numbers which satisfied these properties were only power of 3, then just summed up - 1+3+9+27+81+243+729=1093

Pengu Ultimatrix
Sep 10, 2014

These are simply powers of 3...

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