The phrase "are considered difficult to" is correct and usable in written English.
It can be used when discussing perceptions or opinions about the level of difficulty associated with a task, subject, or concept.
Example: "These mathematical problems are considered difficult to solve without prior knowledge of advanced techniques."
Alternatives: "are viewed as challenging to" or "are seen as hard to".
Min max problems are considered difficult to solve, specially constrained min max optimization problems.
In addition, it provides a new concept for plating the metals, which are considered difficult to plate due to high reactivity.
A new class of PNs, Buffer-nets, for defining FMS is proposed, which enhances the modelling techniques for manufacturing systems with features that are considered difficult to model.
But Mr. Lewellen rejected the notion that the political content of "The Manchurian Candidate" had an effect on its performance, though traditionally political movies are considered difficult to translate into financial success.
This technique is very popular despite their limited precision, because human movements are considered difficult to track by its degree of freedom.
They have often provided loving homes for children who are considered difficult to place...
"Since I was considered difficult to sack, I could be bloody-minded.
But the gas is considered difficult to extract there, requiring wells as deep as 4,000 meters, or over 13,000 feet.
She was considered difficult to please and was said to have turned down memorable parts that went to Gene Tierney ("Laura") and Ingrid Bergman ("Casablanca" and "Gaslight").
Though his work is not widely known in English -- he is considered difficult to translate -- Dario's grip on Spanish-language writers is explained by his interesting bohemian life as well as his literary innovations.
This problem is considered difficult to be dealt with in the control literature, mainly because that the triangular structure of pure-feedback systems has no affine appearance of the variables to be used as virtual controls.
