Finding the inverse function, is there a technique? I came across a way to find whether some number is inside a sequence of numbers.
For example the sequence (simple function for positive odd numbers):
$$a(n) = 2n + 1.$$
So the numbers inside it go: $1, 3, 5, 7, \ldots$
If I want to test if the sequence contains a number $N,$ I reverse the formula to this: $$n = (N - 1) / 2.$$
If $n$ is an integer, then we know that $N$ is inside the sequence.

But, here is my question, how can I find the inverse function of this formula:
  $$a(n) = n(2n - 1)$$


Note: Please do comment every step and explain the logic behind it, so I'll understand
 A: (Note: there is some fine print about domains and well-defined inverses, but I'll skip to the computations)
Completing the square is a method for isolating a variable in a quadratic equation. For example, if we were trying to solve $$x^2 + 4x = 5$$ we could add $4$ to both sides to get $$x^2 + 4x + 4 = 9$$ and then recognize the left hand side as a perfect square trinomial. 
So we could write $$(x+2)^2 = 9$$ and take square roots to get $$x + 2 = \pm 3 \Rightarrow x = -2 \pm 3$$
This gives us TWO values, so we won't have a "well-defined" function unless we restrict the domain. 
In your case, you have $a = 2n^2 - n$, or $a/2 = n^2 - n/2$, and we'll add $\dfrac{1}{16}$ to both sides to get $$\dfrac{a}{2} + \dfrac{1}{16} = (n - \dfrac{1}{4})^2$$
Now take the square root and add $\dfrac{1}{16}$ to get $n$. (Insert comment on domain restrictions here)
A: So what you're describing a situation where any element of the sequence is defined as:$$ a_n = p(n), $$ where $p(n) \in \Bbb Z[n],$ i.e. a polynomial in $n$ with integer coefficients. Now to test that a particular integer $N$ is in the sequence it suffices to show that there exists some integer $n$ such that $p(n) - N = 0.$ This is equivalent to saying that the polynomial $p(n) - N$ has an integer roots.
This test for integer roots can be done by the rational root theorem. Equivalently, you can find the roots for the polynomial equation $p(n) - N = 0,$ e.g. by factoring, and then check whether any of the roots is an integer.
A: If $a$ is your test value then $a = n(2n-1)$ is a quadratic in $n$ with the solution 
$$n = \dfrac{1 \pm \sqrt{8a+1}}{4}$$ 
and you will want this to be a positive integer if your test value is positive.
