This is the first word in the name of a family of sets that can be constructed by recursively gluing together cells of increasing even dimension. It’s not “outer,” but an operation named for this word on a vector space V is equivalent to taking the direct sum of two copies of V, or the direct product of V with a set named for this word. Wirtinger derivatives are used in a subfield of mathematics described by this adjective. Multivalued functions on a set described by this adjective may exhibit branch cuts. A stereographic (10[1])projection maps the Riemann sphere to a plane described by “extended” and this adjective. A subfield of mathematics (10[1])named for this adjective studies contour integration and holomorphic functions. For 10 points, what adjective describes numbers of the form “a plus ib”? ■END■