50 upvotes problem!

Calculus Level 3

Find the number of functions f : R R + f: \mathbb {R\to R^+} such that f ( f ( x ) ) f(f(x)) = f ( x ) f'(x) .


The answer is 0.

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1 solution

Rajdeep Brahma
Jun 9, 2018

f ( x ) f(x) > > 0,So f ( f ( x ) ) > 0 f(f(x))>0 ,So f ( x ) > 0 f'(x)>0 ,Hence f(x) is an increasing function.Now apply mean value theorem, f ( c ) f'(c) = f ( a ) f ( 0 ) a \frac{f(a)-f(0)}{a} for some c in between (a,0),a(f(f(c)))=f(a)-f(0).Now since f(x) is increasing,it is very easy to conclude that f(0)<f(f(c)).Plug this in the previous equation and u get f(a)<(a+1)f(f(c))...So any contradiction?????What if a=(-5)...think it is almost done....Hope u liked the problem....For getting more such problems go on upvoting me...XD.. :P

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