Is "X is known difficult to implement" correct?

grammaticality

The only one of your choices that is a complete sentence is this:

X is known to be difficult to implement.

And there are verbs which fit the other structure, such as find, with which the to be is unnecessary:

X has been found [to be] difficult to implement.

Know with that usage, when it has ever occurred, has been archaic (He knew the climb dangerous).

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