What should be assumed when letters or numbers are written together without any sign or symbol between them?

When letters or numbers are written together without any sign or symbol between them, it is generally assumed that they represent multiplication. This convention is particularly common in algebra and mathematics, where the juxtaposition of variables or numbers implies that they are to be multiplied. For example, in the expression (xy), it is understood that (x) is multiplied by (y).

This notational system helps streamline communication, allowing mathematicians to convey complex expressions without cluttering them with symbols. Multiplication is the assumed operation due to this convention, especially in contexts where it is clear that no other operation is intended.