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alemán árabe búlgaro checo chino coreano croata danés eslovaco esloveno español estonio farsi finlandés francés griego hebreo hindù húngaro indonesio inglés islandés italiano japonés letón lituano malgache neerlandés noruego polaco portugués rumano ruso serbio sueco tailandès turco vietnamita

definición - PAIRING FUNCTION

definición de PAIRING FUNCTION (Wikipedia)

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Wikipedia

Pairing function

                   

In mathematics a pairing function is a process to uniquely encode two natural numbers into a single natural number.

Any pairing function can be used in set theory to prove that integers and rational numbers have the same cardinality as natural numbers. In theoretical computer science they are used to encode a function defined on a vector of natural numbers f:NkN into a new function g:NN.

Contents

  Definition

A pairing function is a primitive recursive bijection

\pi:\mathbb{N} \times \mathbb{N} \to \mathbb{N}.

  Cantor pairing function

  The Cantor pairing function assigns one natural number to each pair of natural numbers

The Cantor pairing function is a pairing function

\pi:\mathbb{N} \times \mathbb{N} \to \mathbb{N}

defined by

\pi(k_1,k_2) := \frac{1}{2}(k_1 + k_2)(k_1 + k_2 + 1)+k_2.

When we apply the pairing function to k_1 and k_2 we often denote the resulting number as \langle k_1, k_2 \rangle \,.

This definition can be inductively generalized to the Cantor tuple function

\pi^{(n)}:\mathbb{N}^n \to \mathbb{N}

as

\pi^{(n)}(k_1, \ldots, k_{n-1}, k_n) := \pi ( \pi^{(n-1)}(k_1, \ldots, k_{n-1}) , k_n) \,.

  Inverting the Cantor pairing function

Suppose we are given z with

 z = \langle x, y \rangle = \frac{(x + y)(x + y + 1)}{2} + y

and we want to find x and y. It is helpful to define some intermediate values in the calculation:

 w = x + y \!
 t = \frac{w(w + 1)}{2} = \frac{w^2 + w}{2}
 z = t + y \!

where t is the triangle number of w. If we solve the quadratic equation

 w^2 + w - 2t = 0 \!

for w as a function of t, we get

 w = \frac{\sqrt{8t + 1} - 1}{2}

which is a strictly increasing and continuous function when t is non-negative real. Since

 t \leq z = t + y < t + (w + 1) =  \frac{(w + 1)^2 + (w + 1)}{2}

we get that

 w \leq \frac{\sqrt{8z + 1} - 1}{2} < w + 1

and thus

 w = \left\lfloor \frac{\sqrt{8z + 1} - 1}{2} \right\rfloor .

So to calculate x and y from z, we do:

 w = \left\lfloor \frac{\sqrt{8z + 1} - 1}{2} \right\rfloor
 t = \frac{w^2 + w}{2}
 y = z - t \!
 x = w - y \!.

Since the Cantor pairing function is invertible, it must be one-to-one and onto.

  References

   
               

 

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