Vice vs Versa

Calculus Level 5

For y = x + 1 y = x+1 , there exists values of x x and y y such that x y = y x x^y = y^x . Find the value of 1 0 6 x 10^6 \cdot x to the nearest integer.


The answer is 2293166.

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1 solution

Ron Gallagher
May 13, 2019

The equation can be written as x^(x+1) = (x+1)^x. A solution to this equation will also be a root of f(x) = ln(x) / x - ln(x+1)/(x+1). Since f(1) < 0 and f(e) >0, there is a root between 1 and e. Using Newton's Method with an initial guess of 2 converges quickly.

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