# Re: [R] Re : Generate a random bistochastic matrix

From: Duncan Murdoch <murdoch_at_stats.uwo.ca>
Date: Mon 16 Oct 2006 - 16:57:45 GMT

On 10/16/2006 11:50 AM, Rolf Turner wrote:
> I don't think this idea has been suggested yet:
>
> (1) Form all n! n x n permutation matrices,

> say M_1, ..., M_K, K = n!.
>
> (2) Generate K independent uniform variates

> x_1, ..., x_k.
>
> (3) Renormalize these to sum to 1,

> x <- x/sum(x)
>
> (4) Form the convex combination

>
> M = x_1*M_1 + ... + x_K*M_K
>
> M is a ``random'' doubly stochastic matrix.
>
> The point is that the set of all doubly stochastic matrices
> is a convex set in n^2-dimensional space, and the extreme
> points are the permutation matrices. I.e. the set of all
> doubly stochastic matrices is the convex hull of the the
> permuation matrices.
>
> The resulting M will *not* be uniformly distributed on this
> convex hull. If you want a uniform distribution something
> more is required. It might be possible to effect uniformity
> of the distribution, but my guess is that it would be a
> hard problem.

But using your solution a Markov chain converging to the uniform distribution would be easy:

Start at a bistochastic matrix X, e.g. at a permutation matrix drawn at random, or at constant 1/n, or whatever.

Choose a direction D at random by drawing two permutation matrices and taking the difference.

Calculate the range of scalar lambda such that X + lambda*D remains stochastic (no negatives, no values greater than 1).

Draw lambda uniformly on this range, and move there.

This is a hit-and-run sampler. I suspect it would mix pretty rapidly. I'd guess a Propp-Wilson perfect version would be fairly easy to put together.

Duncan Murdoch

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