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The product of two CW complexes can be made into a CW complex. Specifically, if X and Y are CW complexes, then one can form a CW complex X × Y in which each cell is a product of a cell in X and a cell in Y, endowed with the weak topology. The underlying set of X × Y is then the Cartesian product of X and Y, as expected.
The product of any collection of compact spaces is compact. (This is Tychonoff's theorem, which is equivalent to the axiom of choice.) In a metrizable space, a subset is compact if and only if it is sequentially compact (assuming countable choice) A finite set endowed with any topology is compact.
An empty product is then given by the limit with respect to the empty category, which is the terminal object of the category if it exists. This definition specializes to give results as above. For example, in the category of sets the categorical product is the usual Cartesian product, and the terminal object is a singleton set.
Thus, the zero-product property holds for any subring of a skew field. If is a prime number, then the ring of integers modulo has the zero-product property (in fact, it is a field). The Gaussian integers are an integral domain because they are a subring of the complex numbers.
The empty product on numbers and most algebraic structures has the value of 1 (the identity element of multiplication), just like the empty sum has the value of 0 (the identity element of addition). However, the concept of the empty product is more general, and requires special treatment in logic, set theory, computer programming and category ...
A function that, given a set of non-empty sets, assigns to each set an element from that set. Fundamental in the formulation of the axiom of choice in set theory. choice negation In logic, an operation that negates the principles underlying the axiom of choice, exploring alternative set theories where the axiom does not hold. choice set
A table can be created by taking the Cartesian product of a set of rows and a set of columns. If the Cartesian product rows × columns is taken, the cells of the table contain ordered pairs of the form (row value, column value). [4] One can similarly define the Cartesian product of n sets, also known as an n-fold Cartesian product, which can be ...
Another example: An empty product (that is, is the empty set) is the same as a terminal object, and some categories, such as the category of infinite groups, do not have a terminal object: given any infinite group there are infinitely many morphisms , so cannot be terminal.