How do I find the inverse of this problem?

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Actually Welcome to the Concept of the Inverse of real function.
Let's consider here, g(n) = y ,
so we get as,
[tex]y = \sqrt[3]{ \frac{n - 1}{2} } [/tex]
no, cubing the power both side we get as,
=>
[tex] {y}^{3} = \frac{n - 1}{2} [/tex]
now,
[tex]2 {y}^{3} = n - 1[/tex]
so finally, we get as,
=>
[tex]n = 2 {y}^{3} + 1[/tex]
hence,here, n = inverse of the g(n) function.
so,
g^-1 (n) = 2y^3+1.