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单词 ProofOfGrowthOfExponentialFunction
释义

proof of growth of exponential function


In this proof, we first restrict to when x and a are integersand only later lift this restricton.

Let a>0 be an integer, let b>1 be real, and let x be aninteger.

Consider the following inequality

(1+1x)a1+ax(1+1x)a-1

If x2, then we have

(1+1x)a1+ax(32)a-1.

Define X to be the greater of 2 and a(3/2)a-1/(1-b); when x>X, we have

(1+1x)ab.

Rewrite xa/bx as follows when x>X:

xabx=XabXn=Xx(1+1n)a1b

By the inequality established above, each term in the productMathworldPlanetmathPlanetmath will bebounded by 1/b, hence

xabxXabX1(b)x-X

Since b>1, it is also the case that b>1, hence we havethe inequality

(b)n1+n(b-1)

Combining the last two inequalities yields the following:

xabxXabX11+(x-X)(b-1)

From this, it follows that limxxa/bx=0 whena and x are integers.

Now we lift the restrictionPlanetmathPlanetmathPlanetmath that a be an integer. Since the powerfunction is increasing, xa/bxxa/bx, so we have limxxa/bx=0 for real valuesof a as well.

To lift the restriction on x, let us write x=x1+x2 wherex1 is an integer and 0x2<1. Then we have

xabx=x1abx1(x1+x2x1)ab-x2

If x>2, then (x1+x2)/x2<1.5. Since x20,b-x21. Hence, for all real x>2, we have

xabx1.5ax1abx1

From this inequality, itfollows that limxxa/bx=0 for real values ofx as well.

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更新时间:2025/5/4 22:39:51