{"action":"created","issue":{"url":"https://api.github.com/repos/sympy/sympy/issues/11342","repository_url":"https://api.github.com/repos/sympy/sympy","labels_url":"https://api.github.com/repos/sympy/sympy/issues/11342/labels{/name}","comments_url":"https://api.github.com/repos/sympy/sympy/issues/11342/comments","events_url":"https://api.github.com/repos/sympy/sympy/issues/11342/events","html_url":"https://github.com/sympy/sympy/issues/11342","id":163599597,"number":11342,"title":"BUG: pde_separate success can depend on order of separated functions","user":{"login":"gdmcbain","id":1588947,"avatar_url":"https://avatars.githubusercontent.com/u/1588947?v=3","gravatar_id":"","url":"https://api.github.com/users/gdmcbain","html_url":"https://github.com/gdmcbain","followers_url":"https://api.github.com/users/gdmcbain/followers","following_url":"https://api.github.com/users/gdmcbain/following{/other_user}","gists_url":"https://api.github.com/users/gdmcbain/gists{/gist_id}","starred_url":"https://api.github.com/users/gdmcbain/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/gdmcbain/subscriptions","organizations_url":"https://api.github.com/users/gdmcbain/orgs","repos_url":"https://api.github.com/users/gdmcbain/repos","events_url":"https://api.github.com/users/gdmcbain/events{/privacy}","received_events_url":"https://api.github.com/users/gdmcbain/received_events","type":"User","site_admin":false},"labels":[],"state":"open","locked":false,"assignee":null,"assignees":[],"milestone":null,"comments":1,"created_at":"2016-07-04T04:05:08Z","updated_at":"2016-10-14T16:06:48Z","closed_at":null,"body":"Consider the two-dimensional heat equation\r\n\r\n    from sympy import Eq, Function, pde_separate, Derivative as D\r\n    from sympy.abc import x, y, t\r\n\r\n    u, U, T = map(Function, 'uUT')\r\n\r\n    eq = Eq(D(u(x, y, t), x, 2) + D(u(x, y, t), y, 2), D(u(x, y, t), t, 2))\r\n    functions = [T(t), U(x, y)]\r\n\r\nThen\r\n\r\n    pde_separate(eq, u(x, y, t), functions)\r\n\r\nworks, returning\r\n\r\n    [Derivative(T(t), t, t)/T(t), Derivative(U(x, y), x, x)/U(x, y) + Derivative(U(x, y), y, y)/U(x, y)]\r\n\r\n(the classical space-time separation of variables) but \r\n\r\n    pde_separate(eq, u(x, y, t), functions[::-1])\r\n\r\nfails, returning `None`.\r\n\r\nAs there's nothing in the documentation about a significance of the order of the argument `sep` of `pde_separate(eq, fun, sep)`, I assume that this is a bug.\r\n"},"comment":{"url":"https://api.github.com/repos/sympy/sympy/issues/comments/253845980","html_url":"https://github.com/sympy/sympy/issues/11342#issuecomment-253845980","issue_url":"https://api.github.com/repos/sympy/sympy/issues/11342","id":253845980,"user":{"login":"nicoguaro","id":1097787,"avatar_url":"https://avatars.githubusercontent.com/u/1097787?v=3","gravatar_id":"","url":"https://api.github.com/users/nicoguaro","html_url":"https://github.com/nicoguaro","followers_url":"https://api.github.com/users/nicoguaro/followers","following_url":"https://api.github.com/users/nicoguaro/following{/other_user}","gists_url":"https://api.github.com/users/nicoguaro/gists{/gist_id}","starred_url":"https://api.github.com/users/nicoguaro/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/nicoguaro/subscriptions","organizations_url":"https://api.github.com/users/nicoguaro/orgs","repos_url":"https://api.github.com/users/nicoguaro/repos","events_url":"https://api.github.com/users/nicoguaro/events{/privacy}","received_events_url":"https://api.github.com/users/nicoguaro/received_events","type":"User","site_admin":false},"created_at":"2016-10-14T16:06:48Z","updated_at":"2016-10-14T16:06:48Z","body":"I think that this order just matters when you have separation functions of multiple variables."}}
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