General Writing and Grammar Help/verb-subject agreement
A mathematics word problem began: "On an island there is a number of outposts."
Everybody I've asked said it should be "On an island there are a number of outposts," because the verb refers to "outposts."
I think the sentence is correct as written, because the verb refers to "a number," which itself is singular, even though it applies to a plural object. Similarly, "a group" is singular but means more than one.
To which word does the verb refer?
The word there, in this sentence, is called an "expletive." Many grammar books conclude that "there" has no grammatical relationship to any part of the sentence. In diagramming sentences,
"there" is placed on a horizontal line ABOVE the sentence line.
You can "test" the sentence by removing the word "there" and rearranging the words.
A number of outposts is/are on the island.
That brings us to the heart of your question. Is the verb in agreement with "number" or with "outposts"? The "natural" way of viewing the sentence is to accept "common usage." If we do, then we would use "are," because of the word "outposts." That is the viewpoint held by the people to whom you talked.
HOWEVER, the word "outposts" is the object of the preposition "of," and objects of prepositions CANNOT be subjects.
Remove the prepositional phrase "of outputs" and you are left with "A number IS on the island."
You are absolutely correct in your explanation of the sentence. Your friends are wrong.
I have just given you additional "ammunition." "Number" is the singular subject and it requires a singular verb, "is."
Please remember to evaluate my answer.