D is a protected function. Use a lower case variable name to avoid clashing with built-in functions (which always start with capital letters. https://reference.wolfram.com/language/ref/D.html
– Chris DegnenAug 05 '20 at 15:32
If you like to represent your matrices with capital letters, you can use script, gothic, or double-struck letters by typing Esc scD Esc for script, goD for gothic, or dsD for double-struck. As Chris mentioned, there are enough single-letter function names built-in that it's best to get used to avoiding them. Not all of them will cause problems right away, but C, D, E, I, K, N, and O are all used internally for various things. D, E, I, and N are functions and will always cause problems. Also, you can't directly use underscores in variable names.
– MassDefectAug 05 '20 at 20:54
Dis a protected function. Use a lower case variable name to avoid clashing with built-in functions (which always start with capital letters. https://reference.wolfram.com/language/ref/D.html – Chris Degnen Aug 05 '20 at 15:32Esc scD Escfor script,goDfor gothic, ordsDfor double-struck. As Chris mentioned, there are enough single-letter function names built-in that it's best to get used to avoiding them. Not all of them will cause problems right away, butC, D, E, I, K, N,andOare all used internally for various things.D, E, I,andNare functions and will always cause problems. Also, you can't directly use underscores in variable names. – MassDefect Aug 05 '20 at 20:54