Is verbatim copying of short technical phrases without quotes but with citation considered plagiarism?

I disagree with the two existing answers (gerrit and Patric).

In mathematical writing, it is not necessary to put quotation marks around very short fragments of descriptive text where that text is the obvious and natural way to express the idea. For example, if Smith has written a paper whose main result is

Theorem. Every even number is divisible by two.

then it is perfectly acceptable to write

Smith [cite] shows that every even number is divisible by two.

without quotation marks and without clumsy rephrasings such as

Smith [cite] shows that all even numbers have two as a factor.

The significant intellectual contribution of the work you're citing is the theorem itself, not the obvious wording that they used to express it. As you say, anybody who understood the concept would probably choose to phrase it in that way, even if they'd never seen the paper you're citing. Mathematical writing would be completely unreadable if every phrase that had ever appeared before was put in quotation marks. After all, Smith was hardly the first author to talk about even numbers – are we going to accuse him of plagiarism for not acknowledging that the phrase "even number" is a quotation from somebody else?

At our Physics Department we would not consider such a sentence without quotation marks as plagiarism, due it is below a threshold of originality. In technical writing words are less important than in humanities. Of course it is not allowed to copy whole paragraphs, but single sentences and phrases, which describe technical terms or lab procedures are too trivial and too standardized concerning the wording so there is no need for quotations marks.