Let \(n \geq 3\) be an odd number.Show that there is a number in the set,\(\{2^1-1,2^2-1,...2^{n-1}-1\}\),which is divisible by \(n\).

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TopNewestThe question is easy. My proof: If say some of the numbers is divisible by n then we are done. If no then there must be two numbers which must be congruent to the same number x(mod n). Let those two numbers be (2^i-1) and (2^j-1). Then subtract the smaller from the larger and factorize. WLOG let i>j. Then u get 2^j common outside and inside the bracket u get (2^(i-j)-1). Now since n is odd, gcd(n,2^j)=1. So n divides (2^(i-j)-1) and note that this quantity lies in the set and hence you are done :)

Sorry for not using Latex as I am in a hurry right now ! – Shrihari B · 10 months ago

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– Adarsh Kumar · 10 months ago

If I can ask,are you a RMO participant?Log in to reply

– Nihar Mahajan · 10 months ago

Seems like I have got a tough competitor from my state ;)Log in to reply

– Shrihari B · 10 months ago

YesLog in to reply

– Adarsh Kumar · 10 months ago

This year?Log in to reply

– Shrihari B · 10 months ago

YesLog in to reply

– Adarsh Kumar · 10 months ago

Which class?Log in to reply

– Shrihari B · 10 months ago

This is my last year I am in 11thLog in to reply

– Adarsh Kumar · 10 months ago

Ohh Well,best of luck!Log in to reply

– Adarsh Kumar · 10 months ago

That is a nice solution and no problem!Log in to reply