I read about the Citigroup Center in NYC in a travel guide, and one sentence said that this building, designed as a graph-paper, is "architecture become mathematics". What does it mean?
It is a poetic phrase (as is shown by the very literary use of whiz-deletion with become).
As with much poetry, the precise meaning is hard to determine. The literal meaning is something like "This building started off as architecture but has turned into mathematics". Since this is clearly nonsense as an objective, literal statement, it must be poetry.
("Is become" is an archaic form of "has become".)
[For the sake of clarity: I'm not saying that this is an established poetic phrase, or a quotation from anywhere: it's probably newly coined, but it is intended to be taken poetically].