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Studies in Rings generalised Unique Factorisation Rings

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-18-<br />

Proposition 1038.<br />

For a r<strong>in</strong>g R, the follow<strong>in</strong>g conditions are equivalent.<br />

(a) R is simple left Art<strong>in</strong>ian<br />

( b) R is simple right Art i n i a n<br />

( c) R is simple and semisimple<br />

( d) R =M (D), for some positive <strong>in</strong>teger n and<br />

n<br />

some division r<strong>in</strong>g D.<br />

RING OF FRACTIONS<br />

In the theory of commutative r<strong>in</strong>gs, Loc a l Ls a t i on<br />

at a multiplicative set plays a very important role.<br />

Most<br />

important is the idea of a quotient field, without which<br />

one<br />

can hardly imag<strong>in</strong>e the study of <strong>in</strong>tegral doma<strong>in</strong>s.<br />

A very useful technique <strong>in</strong> commutative theory is the<br />

localisation at. a prime ideal, which ~educes many problems<br />

to the study of local r<strong>in</strong>gs and their maximal ideals.<br />

I-Iowever,<br />

this is not the case with non-commutative<br />

r<strong>in</strong>gs. Although the set of nonzero elements is a multiplicative<br />

set <strong>in</strong> any -doma<strong>in</strong>,<br />

we have examples of doma<strong>in</strong>s<br />

which do not possess a division r<strong>in</strong>g of quotientso<br />

It<br />

was <strong>in</strong> 1930, t~dt o. Ore characterised those non-commutative<br />

doma<strong>in</strong>s which possess division r<strong>in</strong>gs of fractions.<br />

In fact,

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