It's not wrong, they just overloaded the = operator for the RuntimeComplexity class ;-P
Jokes aside, in my college we would write something like:
O( f(n) ) = O(n^2) + O(n)
Jokes aside, in my college we would write something like:
O( f(n) ) = O(n^2) + O(n)
In the same way that ceiling(1.2) = ceiling(1.9) is symmetric, even if their contents are not equal.
Naively, one would assume that that implies O( f(n) ) - O(n) = O(n^2), which is not correct. However, O( f(n) ) = O(n^2) + O(n) implies O( f(n) ) = O(n^2), which would have been obvious if proper notation had been used.