The entity for the copyright symbol (©) is a built-in special character, so all you have to do to display it is use this entity:
To display the uppercase greek letter sigma, you must first include the helpchar.ent file (at the top of your help volume with your other entity declarations) as shown here:
<!entity SpecialCharacterEntities FILE "helpchar.ent"> &SpecialCharacterEntities;
Then you can place the following entity reference where the sigma character is to appear:
As with any entity, case is not significant in the entity names for special characters.