A number k ∈ N is said to be stable if there exist k distinct natural numbers < a 1 , a 2 , a 3 , ⋯ , a k > such that each a i ≥ 2 and satisfy the relation. a 1 1 + a 2 1 + a 3 1 + ⋯ + a k 1 = 1
Then, the number of stable numbers is
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what is cmi expected cut off this year bro?
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I have no idea man, did you write it too?
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yes...waiting anxiously for results...
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@Rajdeep Brahma – Did you give JEE, what was your rank?
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@Sravanth C. – in mains 4166
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@Rajdeep Brahma – Nice, how much in advanced?
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@Sravanth C. – not even calculated..bad:P....How was your mains?
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@Rajdeep Brahma – My Mains AIR was 5017. Advanced I'm getting 158.
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@Sravanth C. – my marks will be near about that only...:(
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@Rajdeep Brahma – Hmm, keys have been released just now
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@Sravanth C. – I checked fiitjee keys
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@Rajdeep Brahma – What happened?
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@Sravanth C. – Got into isi for b stat and u?
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@Rajdeep Brahma – Nice, I'm thinking about NIT Surathkal CSE or IIT Bhuvaneshvar Electrical.
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@Sravanth C. – I guess that nit is a better choice...see your wish..
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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!
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@Sravanth C. – Welcome bro!!I was getting ece in nit Warangal but left cos of isi
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@Rajdeep Brahma – Oh I think isi is great.
Let k be stable ∑ a 1 1 =1....Now divide by 2...add 0.5...I mean ∑ 2 a 1 1 + 2 1 =1...so k+1 is also stable(Note all 2 a 1 <2)...Now 1= 2 1 + 2 1 (not acceptable since they must be distinct)= 2 1 + 3 1 + 6 1 (acceptable)...So 3 is a stable no. and by induction any no.>3 is stable.
How did you assume that the term 1 / 2 is not present in ∑ 1 / a i .
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I clarified ur doubt above...see if ok or not?
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I am still not very sure about it. What if you've already used 1 / 2 ? Like you've used 1 / 2 while writing the expansion for 3 being a stable number.
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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
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@Rajdeep Brahma – Oh right. I get it now.
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.
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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Let's begin by considering a few examples. Clearly, the numbers 1 and 2 do not satisfy. But the number 3 satisfies since we can write: 6 1 + 2 1 + 3 1 = 1
Notice that we can split the term 3 1 into two terms 1 2 1 and 4 1 . Which imlplies that even 4 is stable .
Let's carefully observe the splitting that we have performed, 3 ⋅ 2 0 1 = 3 ⋅ 2 2 1 + 2 2 1
There we go! We can split write any fraction of the from 3 ⋅ 2 r 1 into two terms as follows: 3 ⋅ 2 r 1 = 2 r + 2 1 + 3 ⋅ 2 r + 2 1
Hence every number greater than 3 is stable .