Hard sudoku for 6 days ago

Hard sudoku is extremely difficult to solve and practically inaccessible for beginners.

2/3 of the numbers are hidden in hard sudoku. To solve such a puzzle, you will have to stock up on at least half an hour of free time. Most often, hard sudoku cannot be solved the first time. To find the right solution, you will have to make various assumptions that may turn out to be incorrect.