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单词 ExampleOfUsingEisensteinCriterion
释义

example of using Eisenstein criterion


For showing the irreducibility (http://planetmath.org/IrreduciblePolynomial2) of the polynomialPlanetmathPlanetmath

P(x):=x5+5x+11

one would need a prime numberMathworldPlanetmath dividing its other coefficients except the first one, but there is no such prime.  However, a suitable  x:=y+a  may change the situation.  Since  the binomial coefficientsMathworldPlanetmath of(y-1)5 except the first and the last one are divisible by 5 and 111(mod5),  we try

x:=y-1.

Then

P(y-1)=y5-5y4+10y3-10y2+10y+5.

Thus the prime 5 divides other coefficients except the first one and the square of 5 does not divide the constant term of this polynomial in y, whence the Eisenstein criterion says that P(y-1) is irreducible (in the field of its coefficients).  Apparently, also P(x) must be irreducible.

It would be easy also to see that P(x) does not have rational zeroes (http://planetmath.org/RationalRootTheorem).

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更新时间:2025/5/4 22:03:47