πe or eπ


By Anonymous on Tuesday, February 6, 2001 - 11:20 pm :
Show by means of a proof which is greater, πe or eπ .
By Dan Goodman (Dfmg2) on Monday, February 12, 2001 - 10:13 pm :
Here's a sketch of a proof.

Let f(x)= ex - xe , and hence f'(x)= ex -e xe-1 =e( ex-1 - xe-1 )

f'(x)=0 when x=1 or e.

f(0)=1 and f'(0)=1

As x tends to infinity, f(x) tends to infinity.

f(1)=e-1 and f(e)=0.

So, assuming that the only points x>0 where f'(x)=0 are x=1 and x=e, the graph of f looks like: it starts at f=1 when x=0, increases up to f=e-1 when x=1, decreases down to f=0 when x=e, and then increases up to infinity as x increases beyond that.

So, f(x)0 for all x, and f(x)=0 only if x=e. In other words f(π)>0, so that eπ - πe >0, so eπ > πe . The only thing you need to check is that the only positive real zeroes of f'(x) are x=1 and x=e. In fact, eπ 23.14 and πe 22.46.

You might like to try using a similar technique to find which is bigger, e1/e or π1/π .