# Is a vector of symbols used as singular or plural?

I am writing a technical report and having trouble with singular/plural. For example, A is a vector composed by a number of components:

A = [x, y, z]

Then should I used A as plural or singular? Which one of the followings is correct?

1. A is the output signals from [...]

2. A are the output signals from [...]

• A is the vector of signals. Singular agreement of A with vector. – Lawrence Mar 16 '18 at 2:17
• Actually, I need to refer to A multiple times like this in the text. For example, as what you suggest, using "is" in the following text is correct? Thank you both Lawrence and Nigel J. "The system model is defined as [a math equation goes here] where A=[x, y, z] is the model outputs, B=[m, p, q] is the system states." – kien Mar 16 '18 at 2:29
• @kien almost, you probably want to make that last states and outputs singular, so you'd get: "The system model is defined as [a math equation goes here] where A=[x, y, z] is the model output, B=[m, p, q] is the system state." – JJJ Mar 16 '18 at 2:44

Your question is a little confusing because you ask about "a vector of signals," but you don't use those words in your example.

Vector is the key word. A vector of signals is singular, because there's only one vector. If you had two vectors of signals, it would be plural.

Since vector is the key word, you should use it to be clear:

A is the vector of output signals...

If you don't want to do that, use the word represents instead of "is":

A represents the output signals...

I realize this is probably too late to help you but I suggest that when defining A you use "A = [x,y,z] is the vector of outputs..." rather than "A = [x,y,z] is the outputs..." That way both sides of the copula "is" will be singular, not one singular and one plural.