sequences...

Prove that there exist two infinite sequences (an)n≥1 and (bn)n≥1 of positive integers such that the following conditions hold simultaneously:

(i) 1 < a1 < a2 < a3 ....

(ii) an<bn<a2n, for all n ≥1

(iii)an-1 is divides bn-1, for all n≥1;

(iv)a2n-1 divides b2n-1, for all n≥1

I am really sorry , had to edit 1 part -

(iii) made it to bn-1 from bn

7 Answers

1
skygirl ·

arey this came today in RMO naa????

1
varun ·

Yes .. did you do it ? I couldn't..

62
Lokesh Verma ·

an=22n

bn=22n+1

will this work.. i am not very sure though!

1
varun ·

No.. an-1 doesn't divide bn-1

1
varun ·

I got it finally :)

62
Lokesh Verma ·

hmm.. cool.. i was trying this one!

1
ith_power ·

take an>1, ≡1(mod 4),

then bn= (an2 +1)/2

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