Numbers x such that x^2 divides 7^x-1
Haskell, 34 bytes

([x|x&lt;-[1..],mod(7^x-1)(x^2)&lt;1]!!)


This uses 0-based indexing. Usage example: ([x|x&lt;-[1..],mod(7^x-1)(x^2)&lt;1]!!) 30 -> 1140.

It's a direct implementation of the definition. It builds a list of all numbers x and picks the nth.