Pairing function

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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.


A pairing function is a primitive recursive bijection

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

Cantor pairing function[edit]

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}


\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[edit]

Let z \in \mathbb{N} be arbitrary and suppose that  z = \pi(x, y). We will show that there exist unique values x, y \in \mathbb{N} such that

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

and hence that \pi is invertible. 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 .

where  \left\lfloor \, \right\rfloor is the floor function. 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.