Implementing periodic or reflective boundary conditions in emcee

[A.N.]

Hi all,

Is it possible to implement periodic or reflective boundary conditions in emcee?

I understand that where the likelihood is more or less completely uniform in a parameter that there can be pile-up at the edges of the prior bounds; and this may be reduced by setting periodic or reflective boundary conditions. Is this true and if so, is there a straightforward way to implement this in emcee?

Thanks.

[Radek Poleski]

Hi,

I talked with Dan about it some time ago. You just need to use 2 variables (x, y) instead of one (e.g., p that changes from p_min to p_max). Both x and y change from -1 to 1 and p is calculated via:

p = p_min + (p_max - p_min) * (np.arctan2(x, y) + np.pi) / (2. * np.pi)

HTH

Radek

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[David W Hogg]

The key idea is that the stretch move assumes that the parameter space is a vector space that obeys vector addition and subtraction. Reflection and rings don’t generally obey vector addition.

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David W. Hogg — http://cosmo.nyu.edu/hogg/