[Choco31415]

A sequence is easy to verify. Choosing the sequence not so much.

Roughly put that is the certificate definition of being in NP.

[andrewla]

The goal here was to show that it was strictly NP-hard, i.e. harder than any problem in NP.

[Nevermark]

Harder to solve, not necessarily harder to verify?

If I am understanding things right.