Since it is likely that you have several theorems like that, it is sensible to define a suitable environment for them.
If $A$ is an $n\times n$ symmetric matrix, then the following statements hold:
\item $A$ has exactly $n$ real eigenvalues counting for multiplicity;
\item eigenvectors corresponding to distinct eigenvalues are orthogonal;
\item $A$ is orthogonally diagonalizable.
This way, if you change your mind about the typesetting, you can just act in a single place, rather than chasing through your document. For instance, changing
noitemsep would produce
nosep altogether would produce
Some stylistic remarks.
The leading item numbers should be upright rather than italic.
Punctuation should be consistent. If you end the preamble with a colon, then the items should be separated by a semicolon or a comma. Capitalizing the first word in an item is optional, but you must be consistent across the whole document.
Even if “A” is a single letter, you must use
\(A\) because it is a math formula nonetheless.
If you write “eigenvector”, be consistent and also write “eigenvalue”.
“The eigenvector” is (mathematically) wrong: a matrix always has infinitely many eigenvectors relative to a given eigenvalue.