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代数数[Algebraic Number] [2005-9-17]
iamet 发表在 ∑〖数学〗
If r is a root of the polynomial equation

a_nx^n+a_(n-1)x^(n-1)+...+a_1x+a_0==0,(1)

where the a_is are integers and r satisfies no similar equation of degree <n, then r is an algebraic number of degree n. If r is an algebraic number and a_n==1, then it is called an algebraic integer.

Examples of algebraic numbers and their degrees are summarized in the following table.

constantdegree
Conway's constant lambda71
Delian constant 2^(1/3)3
Freiman's constant2
golden ratio phi2
Graham's biggest little hexagon area A10
hard hexagon entropy constant kappa_h24
logistic map 4-cycle onset r_42
logistic map 8-cycle onset r_812
logistic map 16-cycle onset r_(16)120
plastic constant3
Pythagoras's constant sqrt(2)2
silver constant3
silver ratio2
tetranacci constant4
tribonacci constant3
Wallis's constant3

If, instead of being integers, the a_is in the above equation are algebraic numbers b_i, then any root of

b_nx^n+b_(n-1)x^(n-1)+...+b_1x+b_0==0,(2)

is an algebraic number.

If alpha is an algebraic number of degree n satisfying the polynomial equation

(x-alpha)(x-beta)(x-gamma)...==0,(3)

then there are n-1 other algebraic numbers beta, gamma, ... called the conjugates of alpha. Furthermore, if alpha satisfies any other algebraic equation, then its conjugates also satisfy the same equation (Conway and Guy 1996).

Any number which is not algebraic is said to be transcendental. The set of algebraic numbers is denoted A (Mathematica), or sometimes Q^_ (Nesterenko 1999), and is implemented in Mathematica as Algebraics. A number x can then be tested to see if it is algebraic using the command Element[x, Algebraics].


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