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Monday, December 12, 2022

Putnam 2012 B4 generalization

 Let c>0 be a real number and an be a sequence such that a0=1 and an+1=an+ecan

Show that

limn(canlogn)=logc

(log is log to base e)


The Putnam problem was with c=1 and only asked for proof of existence of the limit.



Scroll down for a solution.




Consider bn=ean then we get that b0=e and


bn+1=bne1/(bn)c


We can easily show that an is unbounded (proof by contradiction) and so is bn and thus 1bcn0.

The recurrence for an gives us


bcn+1=bcnec/(bn)c


Expanding the e part we get

bcn+1=bcn(1+cbcn+O(1b2cn))=bcn+c+O(1bcn)

This telescopes to give us

bcnbc0=nc+nk=0O(1bck)

And so

bcnbc0n=c+nk=0O(1bck)n

Thus

bcnnc

Taking logarithms gives the result.

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