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单词 DerivationOfHartleyFunction
释义

derivation of Hartley function


We want to show that the Hartley function log2(n) is the onlyfunction mapping natural numbersMathworldPlanetmath to real numbers that

  1. 1.

    H(mn)=H(m)+H(n) (),

  2. 2.

    H(m)H(m+1) (monotonicity), and

  3. 3.

    H(2)=1 (normalization).

Let f be a function on positive integers that satisfies the above three properties. Using the additivePlanetmathPlanetmath property, it is easy to see that the value of f(1) must be zero. So we want to show that f(n)=log2(n) for all integers n2.

From the additive property, we can show that for any integer nand k,

f(nk)=kf(n).(1)

Let a>2 be an integer. Let t be any positive integer. There is a uniqueinteger s determined by

as2t<as+1.

Therefore,

slog2at<(s+1)log2a

and

st1log2a<s+1t.

On the other hand, by monotonicity,

f(as)f(2t)f(as+1).

Using Equation (1) and f(2)=1, we get

sf(a)t(s+1)f(a),

and

st1f(a)s+1t.

Hence,

|1f(a)-1log2(a)|1t.

Since t can be arbitrarily large, the differencePlanetmathPlanetmath on the lefthand of the above inequality must be zero,

f(a)=log2(a).
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