COMPUTATIONAL PROBLEMS IN ABSTRACT ALGEBRA.
COMPUTATIONAL PROBLEMS IN ABSTRACT ALGEBRA.
COMPUTATIONAL PROBLEMS IN ABSTRACT ALGEBRA.
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386 Hans Zassenhaus<br />
For this purpose we must assign to each polynomial U of E[Xj a sign<br />
function SIGN (U, Z, P) assuming one of the 3 values 1, 0, - 1 such that<br />
the operational rules<br />
mu m + ww, PI) = ww, P>> (13)<br />
wu P))VK 9) = WqI, PI> (14)<br />
if tJ(xj+ V(X) = W(X), U(X)V(X) = T(X),<br />
define an ordered extension E(R(Z, P)) consisting of all symbols U(R(Z, P)),<br />
according to the positivity rule<br />
Utw, m > 0 (15)<br />
if and only if SIGN (U(R(Z, P)))= 1, and the equality definition<br />
Utw, PI> = ww, p>> (16)<br />
if and only if SIGN ((U- V) (R(Z, P))) = 0 when (U- V)(X) = U(X) - V(X);<br />
also the conditions (ll), (12) must be fulfilled.<br />
As usual the expression Z(R(Z, P)) is identified with R(Z, P) when Z(X) =<br />
X.<br />
By definition the complexity of the ordered extension E(R(Z, P)) is 1 more<br />
than the complexity of E.<br />
On the degree level 1 the construction with the desired properties is<br />
simple enough.<br />
The algebraically ordered extensions of Pto be considered are Ffor each<br />
complexity.t If P is a constant polynomial over F, then NR(P) = 0.<br />
If P is the linear polynomial AXfB of F[X], and U is any polynomial of<br />
E[fl, then we have the defining equation<br />
SIGN (U, 1, P) = sign (U(-B/A)), (17)<br />
and the symbol U(R( 1, P)) is canonically identified with U( - B/A)).<br />
Theorems 1-6 will be verified readily in case the degree of P is not<br />
greater than 1. We assume now that D=-1, that all constructions on<br />
the degree level D- 1 of any prescribed complexity can be performed as<br />
specified above, and that Theorems l-6 are demonstrated for polynomials<br />
P of degree smaller than D and for any field (in place of F) that can be constructed<br />
on the ievel D - 1.<br />
We begin with a proof of Rolle’s theorem for polynomials of degree D.<br />
The assumption of Theorem 6, viz.<br />
leads to a factorization<br />
P(A) = 0 = P(B),<br />
P(X) = (X- A)L(X- B)MQ(X)<br />
t As A. Hollkott stresses correctly, in reality we do get new ordered fields even here in<br />
as much as the collection of symbols to be considered expands with increasing complexity.<br />
But in our case a canonical order-preserving isomorphism with Fis set up at each stage.<br />
A real root calculus<br />
with positive exponents L, M such that<br />
Q(A) + 0, Q(B) $I 0,<br />
and certainly the degree of Q is less than D - 1. By the induction assumption<br />
there is an ordered extension of F generated by a root Z? of P satisfying<br />
A-=B-= B such that NR (Q, A, Z?) = 1. It su5ces then to prove Rolle’s<br />
theorem under the additional assumption that there is no root of Q between<br />
A and B. By the between value theorem Q(A)Q(B)=-0. Upon differentiation<br />
we have<br />
P’(S) = (X-A)L-l(X- B)“+(X),<br />
&(X) = (L(X-B)+M(X--4))Q(X)+(X-A)(X-B)&’(X),<br />
$(A) = W -B>QW,<br />
&@I = WB- A)&(B),<br />
&(A)&(B) = --LMtA - Bj”QtA)Q@),<br />
&4$(B) -c 0.<br />
By the between value theorem applied to Q(X) there is an ordered extension<br />
of F generated by a root of Q(X) between A and B. This root also is a<br />
root of P’(X) between A, B.<br />
The mean value theorem follows in the customary way by application<br />
of Rolle’s theorem to the polynomial<br />
P(X)-P(B)(P(X)--P(A))I(B--A)-P(A)tP(X)--P(B))I(A -B>.<br />
We proceed to the proof of the between value theorem for a polynomial<br />
P of degree D. For convenience sake let A-= B.<br />
If at any stage of the ensuing construction we should meet an element R<br />
in an ordered extension E of F that was obtained on the D- 1 level such<br />
that A-= R-= B, P(R)= 0, then the elements U(R) (U cF[Xj) with the operational<br />
rules as defined in E provide the required collection of symbols<br />
forming an ordered extension of F with a root of P between A and B.<br />
It will be assumed in the ensuing construction that this will not happen.<br />
For example, if it should happen that there is a non-trivial factorization<br />
P(X) = M(X)L(X) in E[X] such that both M and L are non-constant, then<br />
either M(A)M(B) -C 0 or L(A)L(B) < 0 so that either M or L will have a<br />
root in an ordered extension of E on the D- 1 level.<br />
Henceforth we assume that we will not meet non-trivial factorizations<br />
of P in E[X& This implies that P is separable, because P/GCD (P, P’) cannot<br />
be a proper divisor of P.<br />
Now let E be an ordered extension of F generated by NR(P’, A, B)<br />
distinct roots R(J), . . . ,R(K) of P’ belonging to [A, B], according to Theorems<br />
2, 3. Let<br />
A = A(0) cf A(1) -== . . . -=c A(S) = B<br />
the set formed by the NR (P’, A, B) roots of P’ belonging to [A, B) and<br />
387