One issue is the vocabulary of the word equivilent.
With poloynomial time problem reductions (which is what is usually used with showig a problem is NP complete, though I too haven't read the paper) equivilent just means something like "an exact solution in A translates to an exact solution in B." Unfortunatly, in general, an approxmimate solution to A doens't translate to an approxmiate solution to B.
Note: This part is me really guessing so please don't take it at face falue.
If I had to guess, I'd say that this is like densest subgraph in the sense that:
The mortgages are the nodes.
High correlation --> Edge connections.
So there might be a straightforward translation of approxmiate solutions. Like I said, just a guess.
With poloynomial time problem reductions (which is what is usually used with showig a problem is NP complete, though I too haven't read the paper) equivilent just means something like "an exact solution in A translates to an exact solution in B." Unfortunatly, in general, an approxmimate solution to A doens't translate to an approxmiate solution to B.
Note: This part is me really guessing so please don't take it at face falue.
If I had to guess, I'd say that this is like densest subgraph in the sense that:
The mortgages are the nodes.
High correlation --> Edge connections.
So there might be a straightforward translation of approxmiate solutions. Like I said, just a guess.