If these were nouns, I would assume "single" fits in between:
> **1/4, 1/3, 1/2, *1*, 2, 3, 4 . . .**

> quarter, third, half, *single* or *one*, double, triple, quadruple . . .

Note that each word has a cardinal as part of its etymology: *quarter* evolved from the Latin *quattuor* ("four"), *double* from Latin *duplus* (“twofold”), and so on.

However, in the case of verbs, is there a word to indicate **multiplying (or dividing) by one** that fits into this pattern? The closest I can think of is [*unify*](http://en.wiktionary.org/wiki/unify), "cause to become one", but it wouldn't make sense in context:
>She *doubled* the number, i.e. multiplied by two.<br>
>She *unified* the number, i.e. multiplied by one.

The word wouldn't necessarily have to have the *mono-* or *uni-* prefix, but the etymological root ought to contain the cardinal for **one** (i.e. "she *maintained* the number" wouldn't work) *in keeping with the pattern*.