Let c>0 be a real number and an be a sequence such that a0=1 and an+1=an+e−can
Show that
limn→∞(can−logn)=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 1bcn→0.
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
bcn−bc0=nc+n∑k=0O(1bck)
And so
bcn−bc0n=c+∑nk=0O(1bck)n
Thus
bcnn→c
Taking logarithms gives the result.
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