Question: This is a [cops-and-robbers](/questions/tagged/cops-and-robbers "show questions tagged 'cops-and-robbers'") puzzle, the cops' thread can be found [here.](https://codegolf.stackexchange.com/questions/107012/hidden-inversions-cops-thread)

Your task will be to find an anagram of the provided programs in the cops' thread that performs its left inverse.

Once you crack an answer post the solution as an answer below and notify the original answerer.

You will be scored on the number of programs you are the first to crack.

Answer: 
Python 3, 80 bytes, [Wolfram](https://codegolf.stackexchange.com/a/107187/32014)
--------------------------------------------------------------------------------

```
from bisect import*
q=int(input())
print(bisect([(h+1)**2 for h in range(q)],q))

```

This was really hard to crack!
I use the [bisect library](https://docs.python.org/3/library/bisect.html), which is included in the Python 3 distribution.
The `bisect` function takes a sorted list and an element, and returns the rightmost index where the element could be inserted to maintain order.
We just give it the length-`q` list of squares starting from `1` and the element `q`.
JavaScript (ES6), 46 bytes, [SLuck49](https://codegolf.stackexchange.com/a/107255/42545)
========================================================================================

**Original (calculates ln(x+1))**

```
x=>Math.log(x+(+String(t=985921996597669)[5]))

```

**Crack**

```
x=>Math[(lg=19979699+55686).toString(9+25)](x)

```

I never would have cracked this if I hadn't realized that the inverse is a [`Math` built-in](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/expm1). `(lg=19979699+55686).toString(9+25)` is just a convoluted way of returning `"expm1"`.