[14], [5], [10], and [9] provide the notation and terminology for this paper. The following propositions are true: 1. PRELIMINARIES (1) For every element z of C holds ||z| | = |z|. (2) For all real numbers x1, y1, x2, y2 holds (x1 + y1i) ·(x2 + y2i) = (x1 · x2 − y1 · y2)+(x1 · y2 + x2 · y1)i. (3) For every real number r holds (r+0i) · i = 0+ri. (4) For every real number r holds |r+0i | = |r|. (5) For every element z of C such that |z | � = 0 holds |z|+0i = z |z|+0i · z. 2. SOME FACTS ON THE FIELD OF COMPLEX NUMBERS Let x, y be real numbers. The functor x+yiCF yielding an element of CF is defined as follows: (Def. 1) x+yiCF = x+yi. The element iCF of CF is defined as follows: (Def. 2) iCF = i. We now state several propositions: (6) iCF = 0+1i and iCF = 0+1iCF.

Four statements equivalent to well-foundedness (well-founded induction, existence of recursively defined functions, uniqueness of recursively defined functions, and absence of descending omega-chains) have been proved in Mizar and the proofs mechanically checked for correctness. It seems not to be widely known that the existence (without the uniqueness assumption) of recursively defined functions implies well-foundedness. In the proof we used regular cardinals, a fairly advanced notion of set theory. The theory of cardinals in Mizar was developed earlier by G. Bancerek. With the current state of the Mizar system, the proofs turned out to be an exercise with only minor additions at the fundamental level. We would like to stress the importance of a systematic development of a mechanized data base for mathematics in the spirit of the QED Project.

[14], [5], [10], and [9] provide the notation and terminology for this paper. The following propositions are true: 1. PRELIMINARIES (1) For every element z of C holds ||z| | = |z|. (2) For all real numbers x1, y1, x2, y2 holds (x1 + y1i) · (x2 + y2i) = (x1 · x2 − y1 · y2) + (x1 · y2 + x2 · y1)i. (3) For every real number r holds (r + 0i) · i = 0 + ri. (4) For every real number r holds |r + 0i | = |r|. (5) For every element z of C such that |z | � = 0 holds |z | + 0i = z |z|+0i · z. 2. SOME FACTS ON THE FIELD OF COMPLEX NUMBERS Let x, y be real numbers. The functor x + yiCF yielding an element of CF is defined as follows: (Def. 1) x + yiCF = x + yi. The element iCF of CF is defined as follows: (Def. 2) iCF = i. We now state several propositions: (6) iCF = 0 + 1i and iCF = 0 + 1iCF.

this paper. 1. Preliminaries Let X be a set. Let us observe that X has non empty elements if and only if: (Def. 1) 0 / # X. We introduce X is without zero as a synonym of X has non empty elements. We introduce X has zero as an antonym of X has non empty elements. Let us note that R has zero and N has zero. Let us note that there exists a set which is non empty and without zero and there exists a set which is non empty and has zero. Let us observe that there exists a subset of R which is non empty and without zero and there exists a subset of R which is non empty and has zero. The following proposition is true (1) For every set F such that F is non empty and #-linear and has non empty elements holds F is centered. Let