Monday, 24 June 2013

abstract algebra - Are algebraic numbers analogous to group elements with finite order?




Would you say that the "elements with finite order" in group theory are analogous to "algebraic numbers" in field theory?



I thought this is the case since requiring an algebraic number α to be the root of a polynomial (i.e. requiring a finite combination of terms in α using + and × to give the identity zero) is like the two-operation equivalent of an element g in group theory having finite order (where there is only one operation × and we require a term in g which is required to give the identity 1).



However I don't think the analogy is quite complete because in the case of the polynomial we are allowed to also multiply powers of α by other elements of the field to achieve the identity.


Answer



There is a general model-theoretic notion of algebraic elements, see here.



If L/K is a field extension, then aL is algebraic over K in the usual sense iff it is algebraic in the sense of model theory (applied to the structure (L,+,,0,1) and the subset of K).




If G is a group, then aG is algebraic over in the sense of model theory iff there is some nZ with gn=1 and {hG:hn=1} is finite. Thus, if G is finite, then n=0 works and every element is algebraic. This concept is more interesting when G is infinite. Then every algebraic element is torsion. The converse probably does not hold.


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