Stable Numbers

A number k N k\in \mathbb N is said to be stable if there exist k k distinct natural numbers < a 1 , a 2 , a 3 , , a k > <a _1, a_2, a_3, \cdots ,a_k> such that each a i 2 a_i \geq 2 and satisfy the relation. 1 a 1 + 1 a 2 + 1 a 3 + + 1 a k = 1 \frac 1{a_1}+\frac 1{a_2}+\frac 1{a_3}+\cdots +\frac 1{a_k}=1

Then, the number of stable numbers is


This problem is a slightly modified version of a problem which had appeared in the CMI entrance examination 2018.

A Finite set containing more than 2 elements Cannot say Only two numbers satisfy 3 3 and 4 4 An Infinite set

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3 solutions

Sravanth C.
May 16, 2018

Let's begin by considering a few examples. Clearly, the numbers 1 1 and 2 2 do not satisfy. But the number 3 3 satisfies since we can write: 1 6 + 1 2 + 1 3 = 1 \frac 16 +\frac 12 +\frac 13 =1

Notice that we can split the term 1 3 \dfrac 13 into two terms 1 12 \dfrac 1{12} and 1 4 \dfrac 14 . Which imlplies that even 4 4 is stable .

Let's carefully observe the splitting that we have performed, 1 3 2 0 = 1 3 2 2 + 1 2 2 \frac 1{3\cdot 2^0}=\frac 1{3\cdot 2^2}+\frac 1{2^2}

There we go! We can split write any fraction of the from 1 3 2 r \dfrac 1{3\cdot 2^r} into two terms as follows: 1 3 2 r = 1 2 r + 2 + 1 3 2 r + 2 \frac 1{3\cdot 2^r}=\frac 1{2^{r+2}}+\frac 1{3\cdot 2^{r+2}}

Hence every number greater than 3 3 is stable .

what is cmi expected cut off this year bro?

rajdeep brahma - 3 years ago

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I have no idea man, did you write it too?

Sravanth C. - 3 years ago

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yes...waiting anxiously for results...

rajdeep brahma - 3 years ago

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@Rajdeep Brahma Did you give JEE, what was your rank?

Sravanth C. - 3 years ago

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@Sravanth C. in mains 4166

rajdeep brahma - 3 years ago

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@Rajdeep Brahma Nice, how much in advanced?

Sravanth C. - 3 years ago

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@Sravanth C. not even calculated..bad:P....How was your mains?

rajdeep brahma - 3 years ago

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@Rajdeep Brahma My Mains AIR was 5017. Advanced I'm getting 158.

Sravanth C. - 3 years ago

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@Sravanth C. my marks will be near about that only...:(

rajdeep brahma - 3 years ago

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@Rajdeep Brahma Hmm, keys have been released just now

Sravanth C. - 3 years ago

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@Sravanth C. I checked fiitjee keys

rajdeep brahma - 3 years ago

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@Rajdeep Brahma What happened?

Sravanth C. - 2 years, 11 months ago

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@Sravanth C. Got into isi for b stat and u?

rajdeep brahma - 2 years, 11 months ago

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@Rajdeep Brahma Nice, I'm thinking about NIT Surathkal CSE or IIT Bhuvaneshvar Electrical.

Sravanth C. - 2 years, 11 months ago

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@Sravanth C. I guess that nit is a better choice...see your wish..

rajdeep brahma - 2 years, 11 months ago

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@Rajdeep Brahma Yeah, that's my top priority if I don't get that, I'll be going for Bhuvaneshvar. Thanks man! And All the best!

Sravanth C. - 2 years, 11 months ago

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@Sravanth C. Welcome bro!!I was getting ece in nit Warangal but left cos of isi

rajdeep brahma - 2 years, 11 months ago

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@Rajdeep Brahma Oh I think isi is great.

Sravanth C. - 2 years, 11 months ago

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@Sravanth C. heard so let's see.........

rajdeep brahma - 2 years, 11 months ago
Rajdeep Brahma
May 28, 2018

Let k be stable \sum 1 a 1 \frac{1}{a1} =1....Now divide by 2...add 0.5...I mean \sum 1 2 a 1 \frac{1}{2a1} + 1 2 \frac{1}{2} =1...so k+1 is also stable(Note all 2 a 1 2a1 <2)...Now 1= 1 2 \frac{1}{2} + 1 2 \frac{1}{2} (not acceptable since they must be distinct)= 1 2 \frac{1}{2} + 1 3 \frac{1}{3} + 1 6 \frac{1}{6} (acceptable)...So 3 is a stable no. and by induction any no.>3 is stable.

How did you assume that the term 1 / 2 1/2 is not present in 1 / a i \sum 1/a_i .

Sravanth C. - 3 years ago

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I clarified ur doubt above...see if ok or not?

rajdeep brahma - 3 years ago

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I am still not very sure about it. What if you've already used 1 / 2 1/2 ? Like you've used 1 / 2 1/2 while writing the expansion for 3 3 being a stable number.

Sravanth C. - 3 years ago

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@Sravanth C. see 1=1/2+1/3+1/6...now as per my algo 1=1/2+1/2(1/2+1/3+1/6)=1/2+1/4+1/6+1/12.....I think this will help u

rajdeep brahma - 3 years ago

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@Rajdeep Brahma Oh right. I get it now.

Sravanth C. - 3 years ago

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@Sravanth C. thanks bro

rajdeep brahma - 3 years ago

See a1<1 so of course 2a1 <2 and 1/2 is not present in 2a1....1/2 may be present in a1 doesn't matter much.

rajdeep brahma - 3 years ago
Praveen Kumar
Jul 14, 2018

1/6 + 1/2 + 1/3 is clearly a soln.....

So 1/2 + 1/2 *(1/2 + 1/3 + 1/6) must also be a soln.... Similarly again add half to the half of previous expression..... So there are infinitly many soln.... Simple😊

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