intractable

Not tractable; not able to be managed, controlled, governed or directed.

And I cannot but expect that this will repeatedly lead to the discovery that an initially intractable problem can be factored after all.

Not able to be solved in polynomial time; too difficult to attempt to solve.

Difficult to deal with, solve, or manage. (of a problem)

Work—bureaucratic work in particular—poses a series of intractable dilemmas that often demand compromises with traditional moral beliefs.

Stubborn; obstinate. (of a person)

Difficult to treat (of a medical condition).