You cannot copyright a word or name in and of itself, so you can't sue someone for having a novel titled 13 so long as their novel is distinct from your own. Trademarks can use certain words or word combinations, but often in a way that is stylized or symbolic of a particular unique style and may include font, coloring, and other unique artistic takes. For example, McDonalds cannot copyright or trademark the letter "M" but it can trademark the "Golden Arches" a unique stylized "M" that they use as signage to at all their locations. If the name is a brand of a certain product such, then the name can be trademarked but only with respect to that product. For example, if the Acme Wash-Master is a dish washing machine they can't sue Ace Wash Master, a unique dog bathing system, for using the name "Wash Master" since it's both styled different (Acme uses a dash between words. ACE uses a space) and non-competitive product lines (most people would not wash dishes in a dog bathing device... and one would certainly hope that no dog owners ever said to themselves "Fido stinks and my tub is busted. The dish washer will do in a pinch!").