Seems impossible. Can determine the pairity you're looking for in constant time, but there's no way around having to scan the whole list for it.

PhoneNumber should not participate in hashcode calculation since it's not an equality member.

"Incomplete literals should be closed on the code end" is a bit unclear. Possibly reword?

An alternative path would be to transform the puzzle into an Exact Cover problem and use Knuth's Algorithm X to quickly find a solution. A pretty understandable writeup can be found here.

Transforms Sudoku puzzle into a Partial Cover problem, creating a 2d "array" of booleans with each column representing a constraint and each row representing the placement of a single digit in a single cell.

Implements Knuth's Algorithm X to find a Partial Cover solution- finding a set of rows for which each column-constraint is satisfied exactly once.

True. I'd estimate it's good until around n=70 or so, though, if you need integer-precise fibbonaci numbers. If the use case only needs a certain number of significant figures, the O(1) direct calculation's pretty useful.