Tuesday, 20 September 2016

proof verification - disproving xlogxinTheta(x2)

So I am trying to disprove the claim that xlogxΘ(x2). Now, for some function f to be Θ of a function g, fΘ(g), means that fO(g)fΩ(g). Now it is intuitive that xlogxO(x2) but xlogx cannot be Big-Omega of x2 and, hence, I am trying to prove that xlogxΩ(x2) by proving the negation of xlogxΩ(x2).



Now, according to the definition of Big-Omega, if xlogxΩ(x2), then c,x0R+,xN,xx0xlogxcx2.



Negating this and trying to prove it, we have; c,x0R+,xN,xx0xlogx<cx2.



Letting c,x0 be positive real numbers, I assume xx0. This automatically fulfills the first part of the and statement. How do I proceed from here to prove xlogx<cx2? Even though it looks intuitive, I am think I am missing something. Is my proof headed in the right direction?

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