MATH 200304202159
Index: neobabylonic nrs , complex nrs , recursive nrs

## Little Mathematical Items

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### The Neobabylonic Numbers

- ( Leo Horowitz , 1987/10/21 )

- The Babylonians, they chose as the base of their calculations numbers like 12,60,360 . It appears that these belong to seven numbers with a special property concerning divisibility .

- Consider the natural numbers 1,2,..
- Definition. An "antiprime number" is a number that has more dividers than all its predecessors .
- Definition. A "neobabylonic number" is an (antiprime)number such that the next bigger antiprime number is (at least) twice as big .
- Theorem. There are exactly seven neobabylonic numbers, being: 1 2 6 12 60 360 2520 .
- The proof can be given without knowledge of number theory .

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### Theory of Complex Numbers

A proof of the "Fundamental Theorem of Algebra" by means of galois theory and 2-sylowgroups .
- (L. Horowitz , 1965 ; conceived in 1959 after following a course of prof. H.D. Kloosterman at Leiden )

- Theorem. If R is the field of real numbers , and if i*i=-1 , then R(i) is algebraically closed .
- The little article has appeared in Nieuw Archief voor Wiskunde (3) , XIV , page 95-96 (1966)
- The same proof has appeared independently as an exercise in a book of E. Artin "Algebra" in 1965 .

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### Theory of Enumerable Sets

Invariance of partial order of Recursive Equivalence Types under finite division .
- ( L. Horowitz, 1965/09/10 communicated to prof A. Nerode )

- Theorem. If n > 0 and A and B are R.E.T.'s then nA > nB is equivalent to A > B .
- It it proved here only using conventional proof-methods , and obtained during a seminar of prof B. van Rootselaar at Amsterdam .
- It appeared in Indagationes Mathematicae Amsterdam , 29 , no 1 (1967) 8 pp
- It has also been proved by A. Nerode using the priority method , to appear in Mathematische Annalen 1966 .
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